Fixes handling of empty `extension` matches

[gretzki]

If results.group('extension') returns None, the extension variable is None and thus, the following string concatination filename + extension will fail.

Happened in my case when I used the script as follows: python objc_dep.py ../../objective-c/ios-project-xy/ -i Pods -x "Spec" > projectXY.dot

A bugfix proposal is #14

[Stunner]

Shouldn't this issue be marked as closed seeing as the referenced pull request has been merged?