Don't pass an empty --extra-index-url through to pip.

If the PYPIEXTRAS env var is empty, we current generate a `pip` command-line like this:

    pip install --extra-index-url  -U -r dev-reqs.txt

This means that it will tread "-U" as an extra index url.  Old versions of pip didn't seem to choke on this, but the recent 1.5 release throws an error when it tries to open that as a URL.

This patch elides the option is it's empty.  I'm going to r=myself and push a new version of MoPyTools since this is breaking builds in the wild.