An expression-body callback that writes nothing renders int P => ; / get => ; (CS1525), the output Validate says it prevents #175

What's wrong

When an ExpressionBodyFactory (or the BodyFactory of an AccessorTemplate.Expression) writes no non-blank lines, TemplateRendering writes the arrow anyway and then a bare semicolon (TemplateRendering.cs#L343-L348):

codeBlocker.Write(" => ");
if (last < 0)
{
	codeBlocker.WriteLine(";");
	return;
}

AccessorTemplate.Validate says it throws for the case that "would render as get => ;" (AccessorTemplate.cs#L115-L135). It only checks for a null BodyFactory, though, so #142's fix doesn't cover a factory that runs and writes nothing.

Repro (at 21ac04c)

new PropertyTemplate { Name = "P", Type = "int", ExpressionBodyFactory = _ => { } }.WriteTo(cb);
// int P => ;

new PropertyTemplate { Name = "P", Type = "int", Getter = AccessorTemplate.Expression(_ => { }) }.WriteTo(cb);
// int P
// {
// 	get => ;
// }

A callback that writes only blank lines, for example when joining an empty list of terms, gives the same result. Neither output compiles (CS1525, invalid expression term ;), and nothing tells the generator author.

Why it matters

Block bodies handle "wrote nothing" on purpose and render { }. An expression body has no valid empty form, so the library hands back code that can never compile. The failure only shows up later, as a compile error in the consumer's generated file, far from the template call that caused it.

Suggested fix / acceptance criteria

• Render the expression into its fragment before writing the declaration. If it has no non-blank lines, throw InvalidOperationException and write nothing, matching the null-factory case.
• Do the same for expression accessors, and extend the AccessorTemplate.Validate and PropertyTemplate.ExpressionBodyFactory docs to cover an empty expression.
• Tests: both repros above throw, and leave the CodeBlocker buffer unchanged.