Use test -v to allow overriding defaults with empty strings

[keras]

I had a problem that I could not set GIT_PROMPT_COMMAND_OK=''. Fixed the problem with this.

[ogr3]

What environments did you test this on?

It totally borks on my OS X installation. I can't find any documentation on where test -v is working. (It worked fine in Ubuntu today for me)

line 37: conditional binary operator expected
line 37: syntax error near `GIT_PROMPT_THEME_NAME'

line 37: `  if [[ ! -v GIT_PROMPT_THEME_NAME ]]; then GIT_PROMPT_THEME_NAME="Default"; fi'

[keras]

Debian Linux

~ $ bash --version
GNU bash, version 4.3.42(1)-release (x86_64-pc-linux-gnu)
Copyright (C) 2013 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>

This is free software; you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

[ogr3]

Ok, I did a bit of research and think that I've found a solution that can handle empty strings, without the -v flag. See #208