Fix validation logic of php:function() callbacks in dom and xsl

Two issues:
- Assumed that at least 1 argument (function name) was provided.
- Incorrect error path for the non-callab

Fix validation logic of php:function() callbacks in dom and xsl

Two issues:
- Assumed that at least 1 argument (function name) was provided.
- Incorrect error path for the non-callable case.

Closes GH-12593.

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