gh-128152: Update Argument Clinic to support empty lines

[StanFromIreland]

This will skip the logic that follows after for empty lines so there should be no error. Assert is now no longer needed.

I don't this a NEWS worthy PR? If it is I can add an entry but I think a Skip NEWS label is just fine.

• Issue: Argument Clinic's [python input] blocks do not support empty lines in comments #128152

[picnixz]

I'm not sure it's the proper approach, although it may work. Empty lines could be incorrect directives depending on the DSL. In addition, please add tests for python input directives and for pure clinic directives to see if something changed.