Revert `PbMap._wrappedMap` empty map value (#904) #1247

Job	Run time
format_analyze; linux; Dart dev; PKG: api_benchmark; `dart format --output=none --set-exit-if-cha...	34s
mono_repo self validate	18s
format_analyze; linux; Dart dev; PKG: benchmarks; `dart format --output=none --set-exit-if-change...	26s
format_analyze; linux; Dart 2.19.0; PKG: protobuf; `dart analyze lib`, `dart analyze test`	24s
format_analyze; linux; Dart dev; PKG: protobuf; `dart format --output=none --set-exit-if-changed ...	24s
format_analyze; linux; Dart dev; PKG: protoc_plugin; `dart format --output=none --set-exit-if-cha...	32s
run_tests; linux; Dart 2.19.0; PKG: protoc_plugin; `./../tool/setup.sh`, `make protos`, `dart test`	39s
run_tests; linux; Dart dev; PKG: protobuf; `dart test`	26s
run_tests; linux; Dart dev; PKG: protoc_plugin; `./../tool/setup.sh`, `make protos`, `dart test`	45s
run_tests; linux; Dart 2.19.0; PKG: protobuf; `dart test`	32s
run_tests; osx; Dart 2.19.0; PKG: protobuf; `dart test`	49s
run_tests; osx; Dart dev; PKG: protobuf; `dart test`	53s
run_tests; windows; Dart 2.19.0; PKG: protobuf; `dart test`	44s
run_tests; windows; Dart dev; PKG: protobuf; `dart test`	41s
run_legacy_tests; linux; Dart 2.19.0; PKG: protoc_plugin; `./../tool/setup.sh`, `make protos`, `d...	37s
	8m 44s

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