Clear result view in Execute SQL tab when running a PRAGMA statement

Clear the result table view in the Execute SQL tab of the main window
when a PRAGMA statement was executed that didn't return any rows. Before
this, running e.g. 'PRAGMA table_info(test)' and then 'PRAGMA
table_info(tset)' would still present the result from the first query
even though the second should have cleared the view.

See issue #151.