Improve diagnostics with empty selections.

Improve diagnostics with empty selections.

Two independent fixes to make empty selections work better:
 - Some empty selections could assert or give internal errors, because
   evaluation of some types of expressions for zero atoms was not
   working.  Fixed by removing the asserts/checks.  The check for empty
   selections should now produce useful diagnostics for these cases.
 - Fix the check for empty selections to work for all types of selection
   option input.  Previously, it only worked if the selection was
   provided on the command line, but not if it was provided from a file
   or through interactive input.

Fixes a problem reported on the gmx-users list.  Only fixed in master
since the problem is only with selections that are nearly always
incorrect.  Also, the empty selections check makes the fix much more
useful there.

Change-Id: Ic3e086e637493fc5fefe4594d4407c5a6c53bc8b