TypeScript client method signatures should mark metadata as optional

[andrewmbenton]

This is an inverse issue to #408

There are a couple of method signatures in the generated TypeScript client that should mark the metadata parameter as optional, to reduce explicitly passing unnecessary null values:

grpc-web/javascript/net/grpc/web/generator/grpc_generator.cc

Lines 652 to 655 in 0ec55aa

	printer->Print(vars,
	"request: $input_type$,\n"
	"metadata: grpcWeb.Metadata | null): "
	"$promise$<$output_type$>;\n\n");

grpc-web/javascript/net/grpc/web/generator/grpc_generator.cc

Lines 670 to 674 in 0ec55aa

	printer->Print(vars,
	"request: $input_type$,\n"
	"metadata: grpcWeb.Metadata | null,\n"
	"callback?: (err: grpcWeb.RpcError,\n"
	" response: $output_type$) => void) {\n");

Making the metadata parameter optional in the method returning a Promise would bring the TypeScript client closer in line with the PromiseClient interface available when generating with import_style=commonjs+dts.