[pigeon] allow empty class

[tarrinneal]

instead of preventing it from being defined, I just made it work

[gemini-code-assist]

Code Review

This pull request adds support for empty data classes in Pigeon across Dart, Java, Kotlin, and Objective-C generators, and bumps the package version to 27.2.0. Feedback on the changes suggests adding a blank line between the constructor and the serialization method in the generated Dart code for empty classes to improve readability.

[gemini-code-assist]

When generating Dart code for an empty class, there is no blank line written between the constructor and the _writeToList method. Adding a blank line here improves the readability of the generated code and aligns with Dart style guidelines for class member spacing.

Suggested change

	if (classDefinition.fields.isNotEmpty) {
	indent.newln();
	for (final NamedType field in getFieldsInSerializationOrder(classDefinition)) {
	addDocumentationComments(indent, field.documentationComments, docCommentSpec);
	final String datatype = addGenericTypes(field.type);
	indent.writeln('$datatype ${field.name};');
	indent.newln();
	}
	}
	if (classDefinition.fields.isNotEmpty) {
	indent.newln();
	for (final NamedType field in getFieldsInSerializationOrder(classDefinition)) {
	addDocumentationComments(indent, field.documentationComments, docCommentSpec);
	final String datatype = addGenericTypes(field.type);
	indent.writeln('$datatype ${field.name};');
	indent.newln();
	}
	} else {
	indent.newln();
	}