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