Allow empty partial-block

[TheConstructor]

I authored a partial-template with optional block-includes and happened to write {{#>mail}}{{/mail}} for the test-mail, as I did not need to adjust the optionals. This led to an exception similar to the one below:

System.InvalidOperationException
Sequence contains no elements
   at System.Linq.ThrowHelper.ThrowNoElementsException()
   at System.Linq.Enumerable.First[TSource](IEnumerable`1 source)
   at HandlebarsDotNet.Compiler.PartialBlockAccumulatorContext.GetAccumulatedBlock() in ..\Handlebars.Net\source\Handlebars\Compiler\Lexer\Converter\BlockAccumulators\PartialBlockAccumulatorContext.cs:line 27
   at HandlebarsDotNet.Compiler.BlockAccumulator.AccumulateBlock(Expression parentItem, IEnumerator`1 enumerator, BlockAccumulatorContext context) in ..\Handlebars.Net\source\Handlebars\Compiler\Lexer\Converter\BlockAccumulator.cs:line 56
   at HandlebarsDotNet.Compiler.BlockAccumulator.ConvertTokens(IEnumerable`1 sequence)+MoveNext() in ..\Handlebars.Net\source\Handlebars\Compiler\Lexer\Converter\BlockAccumulator.cs:line 32
   at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
   at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
   at HandlebarsDotNet.Compiler.BlockAccumulator.Accumulate(IEnumerable`1 tokens, ICompiledHandlebarsConfiguration configuration) in ..\Handlebars.Net\source\Handlebars\Compiler\Lexer\Converter\BlockAccumulator.cs:line 13
   at HandlebarsDotNet.Compiler.ExpressionBuilder.ConvertTokensToExpressions(IEnumerable`1 tokens, ICompiledHandlebarsConfiguration configuration) in ..\Handlebars.Net\source\Handlebars\Compiler\ExpressionBuilder.cs:line 25
   at HandlebarsDotNet.Compiler.HandlebarsCompiler.Compile(ExtendedStringReader source, CompilationContext compilationContext) in ..\Handlebars.Net\source\Handlebars\Compiler\HandlebarsCompiler.cs:line 25
   at HandlebarsDotNet.HandlebarsEnvironment.Compile(TextReader template) in ..\Handlebars.Net\source\Handlebars\HandlebarsEnvironment.cs:line 145
   at HandlebarsDotNet.HandlebarsEnvironment.Compile(String template) in ..\Handlebars.Net\source\Handlebars\HandlebarsEnvironment.cs:line 188
   at HandlebarsDotNet.Handlebars.Compile(String template) in ..\Handlebars.Net\source\Handlebars\Handlebars.cs:line 63
   at HandlebarsDotNet.Test.InlinePartialTests.BasicBlockInlinePartial() in ..

Being new to Handlebars.Net it took me a while to understand what really was wrong. I no know, that I can change the template to {{>mail}}, but the exception isn't really helpful and I don't know why it shouldn't just render. So this pull-request allows block-includes with empty blocks.

On a side-note {{#>mail}}{{/mail}} renders, if no mail-partial is present, while {{>mail}} throws an exception, if no MissingPartialTemplateHandler is registered. While not necessary for my use-case it may even be nice to have a syntax, that allows for optional partials out-of-the-box.

[TheConstructor]

@rexm I rebased on the current master, which disabled the auto-merge you set

[rexm]

CI failure diagnosis

The Issue519_PartialBlockUsableAsBlockAndInIf test fails because of the Expression.Empty() change in PartialBlockAccumulatorContext.GetAccumulatedBlock().

The 0-body case previously returned null, which the partial renderer interpreted as "no fallback — render the outer @partial-block as-is." Changing it to Expression.Empty() gives the renderer a defined (but empty) fallback expression. When {{#> @partial-block }}{{/@partial-block}} appears inside a partial, the empty body is now treated as a new @partial-block definition that overrides the content passed in from the outer call ("Block content"), so the third assertion fails:

Not found: "Block:Block content"
String:    "Conditional: Block content\nPlain: Block content\n..."

Fix: keep null for the 0-body case — this avoids the original InvalidOperationException: Sequence contains no elements crash (which came from .First() on an empty list) while preserving the "no fallback" rendering semantics:

var fallback = _body.Count switch
{
    0 => (Expression)null,   // ← null, not Expression.Empty()
    1 => _body[0],
    _ => Expression.Block(_body)
};

[TheConstructor]

The Issue519_PartialBlockUsableAsBlockAndInIf test fails because of the Expression.Empty() change in PartialBlockAccumulatorContext.GetAccumulatedBlock().

Fix: keep null for the 0-body case — this avoids the original InvalidOperationException: Sequence contains no elements crash (which came from .First() on an empty list) while preserving the "no fallback" rendering semantics:

var fallback = _body.Count switch
{
    0 => (Expression)null,   // ← null, not Expression.Empty()
    1 => _body[0],
    _ => Expression.Block(_body)
};

While it is true, that the test of #519 fails, this is not the fix. If you replaced {{#> @partial-block }}{{/@partial-block}} in said test with
e.g. {{#> @partial-block }}any fallback{{/@partial-block}} it will contain any fallback instead of Block content. I couldn't yet find a way to only override the PartialBlockContent, if we are not in {{#> @partial-block }}, but I will continue tomorrow