qt installwizard: fix empty filename crash
qt installwizard: fix empty filename crash

Traceback (most recent call last):
  File "...\electrum\electrum\gui\qt\installwizard.py", line 256, in on_filename
    widget_create_new.setVisible(temp_storage and temp_storage.file_exists())
TypeError: setVisible(self, bool): argument 1 has unexpected type 'NoneType'

pooler/electrum-ltc,wakiyamap/electrum-mona,vialectrum/vialectrum,wakiyamap/electrum-mona,spesmilo/electrum,vialectrum/vialectrum,spesmilo/electrum,pooler/electrum-ltc,pooler/electrum-ltc,spesmilo/electrum,pooler/electrum-ltc,wakiyamap/electrum-mona,vialectrum/vialectrum,wakiyamap/electrum-mona,spesmilo/electrum