XAS-102 empty string should be allowed
XAS-102 do not throw an exception when the String is wrong, simply empty
the combo.

If you throw an exception then the editor will not open properly and so
the user cannot have a chance to fix it.
RichBeans: an empty string should be a valid entry
Merge branch '8.30' of ssh://dascgitolite@dasc-git.diamond.ac.uk/gda/gda-common-rcp.git into 8.30