Avoid adding empty splices in `CodedBufferWriter` (#886) #1213

Job	Run time
format_analyze; linux; Dart dev; PKG: benchmarks; `dart format --output=none --set-exit-if-change...	31s
format_analyze; linux; Dart dev; PKG: api_benchmark; `dart format --output=none --set-exit-if-cha...	42s
mono_repo self validate	21s
format_analyze; linux; Dart 2.19.0; PKG: protobuf; `dart analyze lib`, `dart analyze test`	36s
format_analyze; linux; Dart dev; PKG: protobuf; `dart format --output=none --set-exit-if-changed ...	32s
format_analyze; linux; Dart dev; PKG: protoc_plugin; `dart format --output=none --set-exit-if-cha...	35s
run_tests; linux; Dart dev; PKG: protobuf; `dart test`	31s
run_tests; linux; Dart 2.19.0; PKG: protoc_plugin; `./../tool/setup.sh`, `make protos`, `dart test`	59s
run_tests; linux; Dart dev; PKG: protoc_plugin; `./../tool/setup.sh`, `make protos`, `dart test`	1m 5s
run_tests; linux; Dart 2.19.0; PKG: protobuf; `dart test`	37s
run_tests; osx; Dart 2.19.0; PKG: protobuf; `dart test`	59s
run_tests; windows; Dart 2.19.0; PKG: protobuf; `dart test`	55s
run_tests; osx; Dart dev; PKG: protobuf; `dart test`	50s
run_tests; windows; Dart dev; PKG: protobuf; `dart test`	55s
run_legacy_tests; linux; Dart 2.19.0; PKG: protoc_plugin; `./../tool/setup.sh`, `make protos`, `d...	55s
	11m 3s

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