Check whether $COLUMNS is empty or zero, not only empty

Fixes #983
Call powerline.new_window early under certain conditions

Fixes #250 for the GUI.
Merge pull request #984 from ZyX-I/fix-250

Call powerline.new_window early under certain conditions
Merge pull request #985 from ZyX-I/fix-983

Check whether $COLUMNS is empty or zero, not only empty