Fix empty macro yield value be nil instead of nop

[straight-shoota]

Fixes #10406

[straight-shoota]

It seems this broke something in the compiler... 👀

[HertzDevil]

Will begin; end work?

[straight-shoota]

I'll try that. Not sure if it helps.

[straight-shoota]

Still fails. But this looks like a parser bug, actually.

[straight-shoota]

Okay the implementation was just wrong because it replaced every Nop when it should only replace Nop in a Yield.

The actual stack overflow is just #10409 and it seems like the stacktrace incorrectly points to the parser as the last frames.

[HertzDevil]

My suggestion to use begin; end is due to #5258, by the way; an empty {{ yield }} would then become an empty Expressions (though the same needs to be done for single-expression blocks too).

[straight-shoota]

That distinction doesn't really matter here because it's never expressed in the AST. It just happens in the method that stringifies the generated macro code. In the result there is no more yield that would be expected to be an Expressions.
I don't care much either way, though.