Fix gRPC-Web incorrect empty response handling #743

Job	Run time
build-library-macos	3m 48s
build-eliza-cocoapods-example	1m 30s
build-library-ios	2m 59s
build-eliza-swiftpm-example	2m 37s
build-library-tvos	3m 19s
build-library-watchos	3m 5s
build-plugin-and-generate	1m 7s
run-conformance-tests	2m 22s
run-swiftlint	9s
run-unit-tests	1m 46s
validate-license-headers	17s
	22m 59s

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