RPC: allow null serviceImpl in BindService

[bangfalse]

For instance, using MapGrpcService from Grpc.AspNetCore.Server invokes this method with a null serviceImpl.

[jamescourtney]

Thanks for your contribution! I will review in the next couple of days and likely approve. This looks like an easy one :)

[codecov]

Codecov Report

Merging #361 (8253098) into main (72a0ecf) will increase coverage by 0.31%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #361      +/-   ##
==========================================
+ Coverage   96.15%   96.46%   +0.31%     
==========================================
  Files         115      114       -1     
  Lines        8742     8691      -51     
  Branches      815      812       -3     
==========================================
- Hits         8406     8384      -22     
+ Misses        230      202      -28     
+ Partials      106      105       -1     

Impacted Files	Coverage Δ
...harp.Compiler/SchemaModel/RpcServiceSchemaModel.cs	98.97% <100.00%> (+0.01%)	⬆️
src/FlatSharp.Runtime/SpanComparers.cs	84.09% <0.00%> (-2.28%)	⬇️
src/FlatSharp.Runtime/IO/SpanWriterExtensions.cs	100.00% <0.00%> (ø)
src/FlatSharp.UnityPolyfills/NativeArray.cs
...atSharp/Serialization/RoslynSerializerGenerator.cs	98.33% <0.00%> (+0.18%)	⬆️
...peModel/Vectors/UnityNativeArrayVectorTypeModel.cs	98.63% <0.00%> (+16.54%)	⬆️

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[jamescourtney]

This looks good. Thank you for your contribution!

If you're up for it, it would be really great to get some gRPC tests that actually use ASP.NET / Kestrel instead of the old-style ones I have now that use the (deprecated) Google gRPC libraries.