Reduce GetTypeByMetadataName allocations for fail-path

[Youssef1313]

Fixes #64142

Per my understanding, this is the root cause of https://github.com/unoplatform/uno/issues/7506

[AlekseyTs]

            result = scope.LookupMetadataType(ref emittedName);

Is this call going to allocate a missing symbol when lookup fails?

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Refers to: src/Compilers/CSharp/Portable/Symbols/NonMissingModuleSymbol.cs:203 in 86aedd9. [](commit_id = 86aedd93cecd94e16ba18fa1d578f82a170035fb, deletion_comment = False)

[AlekseyTs]

                CacheTopLevelMetadataType(ref emittedName, result);

It doesn't look like we want to cache null here.

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Refers to: src/Compilers/CSharp/Portable/Symbols/NonMissingAssemblySymbol.cs:177 in 86aedd9. [](commit_id = 86aedd93cecd94e16ba18fa1d578f82a170035fb, deletion_comment = False)