Delay initialization of column sizes until model contains keys

This ensures that resizing the columns to their contents actually has
some contents to work with. Previously, the columns were resized to the
widths of the column headers if the model was empty.

(cherry picked from commit 6b85d4fa4480dacbfae6c38c7869be128a885a73)

• GnuPG-bug-id: T7082