Revert `PbMap._wrappedMap` empty map value #1246

Job	Run time
format_analyze; linux; Dart dev; PKG: api_benchmark; `dart format --output=none --set-exit-if-cha...	24s
mono_repo self validate	13s
format_analyze; linux; Dart dev; PKG: benchmarks; `dart format --output=none --set-exit-if-change...	21s
format_analyze; linux; Dart 2.19.0; PKG: protobuf; `dart analyze lib`, `dart analyze test`	30s
format_analyze; linux; Dart dev; PKG: protobuf; `dart format --output=none --set-exit-if-changed ...	20s
format_analyze; linux; Dart dev; PKG: protoc_plugin; `dart format --output=none --set-exit-if-cha...	28s
run_tests; linux; Dart dev; PKG: protoc_plugin; `./../tool/setup.sh`, `make protos`, `dart test`	44s
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`	30s
run_tests; osx; Dart dev; PKG: protobuf; `dart test`	54s
run_tests; osx; Dart 2.19.0; PKG: protobuf; `dart test`	1m 4s
run_tests; linux; Dart 2.19.0; PKG: protobuf; `dart test`	33s
run_tests; windows; Dart 2.19.0; PKG: protobuf; `dart test`	59s
run_tests; windows; Dart dev; PKG: protobuf; `dart test`	43s
run_legacy_tests; linux; Dart 2.19.0; PKG: protoc_plugin; `./../tool/setup.sh`, `make protos`, `d...	37s
	8m 59s

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