keepassx2pass: Substitute empty string for None

If keepassx2pass.py is given an xml file containing passwords with an empty
title, like <title></title>, ElementTree.text returns None. This commit
substitutes an empty string; which will produce a password with name '_',
instead of raising AtttributeError, as shown in this exception::

    Traceback (most recent call last):
      File "contrib/importers/keepassx2pass.py", line 80, in <module>
        main(sys.argv[1])
      File "contrib/importers/keepassx2pass.py", line 77, in main
        import_group(group)
      File "contrib/importers/keepassx2pass.py", line 71, in import_group
        import_entry(entry, npath)
      File "contrib/importers/keepassx2pass.py", line 58, in import_entry
        print "Importing " + path_for(element, path)
      File "contrib/importers/keepassx2pass.py", line 37, in path_for
        title = cleanTitle(space_to_camelcase(element.find('title').text))
      File "contrib/importers/keepassx2pass.py", line 16, in space_to_camelcase
        for word in value.split(" "):
    AttributeError: 'NoneType' object has no attribute 'split'
Allow options in EDITOR variable

This is required by most graphical editors, as the shell command they
provide needs to be told to wait until the file has been closed.

This reverts '311c98c9140205f32be2720db9261c67da658c4c'. We thus use
PATH so that the tests still pass in directories with spaces.