[wapiti] Fix bug where a ranking could be None.

Specified a default ranking value to UNKNOWN when trying to retrieve the one
from the signatures database in case the matching fails.
[report] Fix ValueError in get_highest_ranking

Fix ValueError: max() arg is an empty sequence in get_highest_ranking that
could be triggered when self.vulns is empty.
[tools] Fix IndexError in parse().

When calling _recursive_find function, first test if a result has been found
and only then keep the first one.
[doc] Fix typos in docstrings.
[wapiti] Fix typo in docstring.
[doc] Update doc version.