Fix _safe_split function

[cyblue9]

Fixed return value of _safe_split function.

_safe_split function return value is list type, but If an exception occurs, str type is returned.
At that time, get_completions() -> completing_subcommand_option() -> return [](in https://github.com/donnemartin/haxor-news/blob/master/haxor_news/completer.py 184 line).
This is the same as to return [] in completer.py 172 line.
So, If an exception occurs, [] is returned in _safe_split, the behavior does not change!

Please confirm!

[codecov-io]

Codecov Report

Merging #152 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master     #152   +/-   ##
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  Coverage   93.53%   93.53%           
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  Files          31       31           
  Lines        1469     1469           
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  Hits         1374     1374           
  Misses         95       95