genex: Fix preprocessing with incomplete content (#14410).

Similar incomplete generator expressions are already tested
in the GeneratorExpression unit test, but those are executed
with add_custom_target. The generator expressions in the include
directories are run through the preprocessor, whereas the ones
run through add_custom_target are not.
Genex: Test preprocessing incomplete expressions.
Merge topic 'fix-genex-preprocessing-incomplete-test' into next

66fd536 Genex: Test preprocessing incomplete expressions.
ec8ca59 genex: Fix preprocessing with incomplete content (#14410).