numbers_to_digits doesn't handle empty token list

[ChanceNCounter]

Bug revealed by mismatch between Mycroft and a fallback skill's configured languages:

    return CatalanNormalizer().normalize(text, remove_articles)
  File "/home/jmontane/mycroft-core/.venv/lib/python3.7/site-packages/lingua_franca/lang/parse_common.py", line 172, in normalize
    utterance = self.numbers_to_digits(utterance)
  File "/home/jmontane/mycroft-core/.venv/lib/python3.7/site-packages/lingua_franca/lang/parse_common.py", line 119, in numbers_to_digits
    words = self.tokenize(utterance)
  File "/home/jmontane/mycroft-core/.venv/lib/python3.7/site-packages/lingua_franca/lang/parse_ca.py", line 252, in tokenize
    if tokens[-1] == '-':
IndexError: list index out of range

(reported by jmontane in Mattermost)

[JarbasAl]

not related to any mismatch