Fix crash during loading

If the `templatetags` folder exists and is empty, `import_module` will succeed, but `mod` will we be None, then bad things will happen in `mod.__file__`.

This situation can occur in a build server in a git working copy, because git doesn't know about directories, so if a templatetag is added and then removed, the directory will stay empty forever.

I think that in this situation, we should just skip the module, because obviously it contains no template tags. Thoughts ?