Fix in pkgutil: don't call pipes.quote() on None

(resubmission based on #7735)

Although the 'site' parameter is intended to be optional, omitting it would cause the following exception if the requested package was already installed:

    invalid output was: Traceback (most recent call last):
      File "/tmp/ansible-tmp-1402553652.18-229268856436124/pkgutil", line 1398, in <module>
        main()
      File "/tmp/ansible-tmp-1402553652.18-229268856436124/pkgutil", line 153, in main
        if not package_latest(module, name, site):
      File "/tmp/ansible-tmp-1402553652.18-229268856436124/pkgutil", line 78, in package_latest
        site = pipes.quote(site)
      File "/usr/lib/python2.6/pipes.py", line 271, in quote
        for c in file:
    TypeError: 'NoneType' object is not iterable