Apple: Fix _platform_strlen crashes inside getSystemProxyForUri

[lawrencewuskyboxlabs]

I've seen some crashes on iOS where a _platform_strlen error is thrown in the basic_string constructor during getSystemProxyForUri. In general, since UTF8String returns a char pointer, the three locations where it's used should check that the pointer is valid before using it to create a string.

[lawrencewuskyboxlabs]

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