bug#71406: 30.0.50; string-edit signals error when user aborts and :abort-callback parameter nil

[Nicholas Vollmer]

1. emacs -q
2. eval (string-edit "test" "" #'ignore)
3. In the "*edit string*" buffer eval string-edit-abort.

The following error is signaled:

Debugger entered--Lisp error: (void-variable 
string-edit--abort-callback)
 string-edit-abort()
 funcall-interactively(string-edit-abort)
 command-execute(string-edit-abort)

Seeing as string-edit specifies this parameter as a &key, it 
should either provide a default (#'ignore?) or error if the caller 
does not provide an abort function (Though why specify it as a 
keyword arg if it's mandatory?).