R1:a5c93dea568b

Fix InvalidArgumentException on commit hook

Summary:
Fix a regression introduced here:

96ae4ba13acbf0e2f8932e950a92af0495f034d7

I reproduced this exception executing "svn commit" on a hosted repository.

That crash happened because the PHP getenv() function can return false.
But, that is a very terrible value that blasts the non-string-empty check.

So, now the default getenv() value is skipped, without causing problems.

Closes T15253
Ref T15190

Test Plan: - I've run `svn commit` and I have not…