Fix string-width error with empty file

Unlike length, string-width does not return 0 when nil is given.
Create a wrapper function deft-string-width to handle nil argument.

This happens when deft tries to list a file without title or summary,
ie empty file.  A backtrace is like this:

  Debugger entered--Lisp error: (wrong-type-argument stringp nil)
    string-width(nil)
    (min line-width (string-width title))