_checkNotNull field checks for scalars are not necessary (since null safety)

[osa1]

Moving this TODO from #959.

I think "check" functions in PbLists can be completely eliminated for the scalars where we only check for nulls:

protobuf.dart/protobuf/lib/src/protobuf/field_error.dart

Lines 75 to 107 in d00f905

	CheckFunc getCheckFunction(int fieldType) {
	switch (fieldType & ~0x7) {
	case PbFieldType._BOOL_BIT:
	case PbFieldType._BYTES_BIT:
	case PbFieldType._STRING_BIT:
	case PbFieldType._DOUBLE_BIT:
	case PbFieldType._ENUM_BIT:
	case PbFieldType._GROUP_BIT:
	case PbFieldType._MESSAGE_BIT:
	case PbFieldType._INT64_BIT:
	case PbFieldType._SINT64_BIT:
	case PbFieldType._SFIXED64_BIT:
	case PbFieldType._UINT64_BIT:
	case PbFieldType._FIXED64_BIT:
	// We always use the full range of the same Dart type.
	// It's up to the caller to treat the Int64 as signed or unsigned.
	// See: https://github.com/google/protobuf.dart/issues/44
	return _checkNotNull;
	case PbFieldType._FLOAT_BIT:
	return _checkFloat;
	case PbFieldType._INT32_BIT:
	case PbFieldType._SINT32_BIT:
	case PbFieldType._SFIXED32_BIT:
	return _checkSigned32;
	case PbFieldType._UINT32_BIT:
	case PbFieldType._FIXED32_BIT:
	return _checkUnsigned32;
	}
	throw ArgumentError('check function not implemented: $fieldType');
	}

The generated API for these have non-nullable element type (e.g. PbList<int>) so null checks no longer do anything (since null safety).