Enable minimum bounds equal to zero in GUI

The current GUI implementation assumes that there is no bound if the minimum bound is set to zero. 'No bound' should be specified as a separate case so that a minimum bound of zero can be specified.

Start by locating the GUI implementation that handles minimum bounds. Inspect how a minimum value of zero is currently interpreted versus an absent bound. Done means the GUI can represent and submit zero as a valid minimum while still supporting an explicitly unbounded case.

Written by the indexing model from the issue text.