perf: skip ImmutableMetadata allocation when builder has no entries

src/main/java/dev/openfeature/sdk/ImmutableMetadata.java

@@ -101,7 +101,7 @@ public <T> T getValue(final String key, final Class<T> type) {
     }
 
     public Map<String, Object> asUnmodifiableMap() {
-        return Collections.unmodifiableMap(metadata);
+        return metadata.isEmpty() ? Collections.emptyMap() : Collections.unmodifiableMap(metadata);
     }
 
     public boolean isEmpty() {
@@ -199,6 +199,9 @@ public ImmutableMetadataBuilder addBoolean(final String key, final Boolean value
          * Retrieve {@link ImmutableMetadata} with provided key,value pairs.
          */
         public ImmutableMetadata build() {
+            if (metadata.isEmpty()) {
+                return EMPTY;
+            }
             return new ImmutableMetadata(this.metadata);
         }
     }

src/test/java/dev/openfeature/sdk/ImmutableMetadataTest.java

@@ -1,10 +1,12 @@
 package dev.openfeature.sdk;
 
+import static org.assertj.core.api.Assertions.assertThat;
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertNotEquals;
 
 import java.util.Map;
 import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.DisplayName;
 import org.junit.jupiter.api.Test;
 
 class ImmutableMetadataTest {
@@ -38,4 +40,18 @@ void retrieveAsUnmodifiableMap() {
         assertEquals(unmodifiableMap.get("key1"), "value1");
         Assertions.assertThrows(UnsupportedOperationException.class, () -> unmodifiableMap.put("key3", "value3"));
     }
+
+    @Test
+    @DisplayName("empty builder returns EMPTY singleton")
+    void emptyBuilderReturnsEmptySingleton() {
+        assertThat(ImmutableMetadata.builder().build()).isSameAs(ImmutableMetadata.EMPTY);
+    }
+
+    @Test
+    @DisplayName("asUnmodifiableMap on empty metadata returns empty unmodifiable map")
+    void asUnmodifiableMapOnEmptyMetadataIsUnmodifiable() {
+        Map<String, Object> map = ImmutableMetadata.EMPTY.asUnmodifiableMap();
+        assertThat(map).isEmpty();
+        Assertions.assertThrows(UnsupportedOperationException.class, () -> map.put("key", "value"));
+    }
 }