Fix parsing of deprecated list options failing with empty strings

[ankostis]

When a list trait is aliased, and a single empty-string is given in command-line, like this:

foo -=seq-option=""

The traitlets.config.loader fails when attempting to parse it as [deprecated, list, format], with this error:

  File "/home/user/traitlets/config/application.py", line 702, in parse_command_line
    self.cli_config = deepcopy(loader.load_config())
  File "/home/user/traitlets/config/loader.py", line 804, in load_config
    self._convert_to_config()
  File "/home/user/traitlets/config/loader.py", line 947, in _convert_to_config
    self._exec_config_str(k, v, trait=trait)
  File "/home/user/traitlets/config/loader.py", line 542, in _exec_config_str
    (r[0] == '[' and r[-1] == ']') or
IndexError: string index out of range

• The fix consists of skipping deprecated-list parsing on zero-length strings.
• A TC is included proving the problem is fixed.