Argument Clinic silently allow empty custom C basenames

[erlend-aasland]

See #107964 (comment):

Here's a very contrived example that silently passes on this PR, current main and on 3.12:

/*[clinic input]
output everything block
class C "void *" ""
# Trailing whitespace on following line!!!
C.meth as 
[clinic start generated code]*/

Linked PRs

• gh-107972: Argument Clinic: Ensure a C basename is provided after 'as' #107973

[erlend-aasland]

We should fail() with something like "No C basename provided after 'as' keyword" in this case.