Empty UPOS should block running higher-level tests

[dan-zeman]

Otherwise the validator will crash.

[Line 1694 Sent 13_19_stol-054_artik_1554-p8-s1]: [L2 MORPHO unknown-upos] Unknown UPOS tag: '_'.
Traceback (most recent call last):
  File "/net/work/people/zeman/unidep/tools/validate.py", line 33, in 
    errcode = main()
              ^^^^^^
  File "/net/work/people/zeman/unidep/tools/validate.py", line 22, in main
    state = validator.validate_files(args.input)
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/lnet/work/people/zeman/unidep/tools/udtools/src/udtools/validator.py", line 155, in validate_files
    self.validate_file(filename, state)
  File "/lnet/work/people/zeman/unidep/tools/udtools/src/udtools/validator.py", line 190, in validate_file
    self.validate_file_handle(inp, state)
  File "/lnet/work/people/zeman/unidep/tools/udtools/src/udtools/validator.py", line 217, in validate_file_handle
    self.validate_sentence(lines, state)
  File "/lnet/work/people/zeman/unidep/tools/udtools/src/udtools/validator.py", line 323, in validate_sentence
    self.check_upos_vs_deprel(state, node)
  File "/lnet/work/people/zeman/unidep/tools/udtools/src/udtools/level3.py", line 325, in check_upos_vs_deprel
    if deprel == 'mark' and re.match(r"^(NOUN|PROPN|ADJ|PRON|DET|NUM|AUX|INTJ)", upos):
                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/lnet/work/people/zeman/python-env/lib/python3.11/site-packages/regex/regex.py", line 254, in match
    return pat.match(string, pos, endpos, concurrent, partial, timeout)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: expected string or buffer