Fix a regression introduced here:
96ae4ba13acbf0e2f8932e950a92af0495f034d7
I reproduced this exception while using Subversion 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.
We know. Ideally, that check should just says "hey, nope, this is not a
non-empty string" - but - at the moment that check explodes so violently
for non-strings.
So, now the default getenv() value is skipped, without causing problems.