Fix check wxGUI Set vector output format native format dsn

[tmszi]

Reproduce:

1. Open Set vector output format dialog

Error message appear in the Console page:

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/wx/core.py", line
3190, in <lambda>

lambda event: event.callable(*event.args, **event.kw) )
  File
"/usr/local/grass79/gui/wxpython/gui_core/gselect.py", line
1662, in _postInit

dsn = os.path.expandvars(dsn)  # v.external.out uses $HOME
  File "/usr/lib/python3.6/posixpath.py", line 288, in
expandvars

path = os.fspath(path)
TypeError
:
expected str, bytes or os.PathLike object, not NoneType