Allowing groupsideblockmaxgroups to be set as blank (Bug #1377371)

Also allow a text field to change from 0 to blank. Currently it doesn't
update the user even though they are different.

Change-Id: I4dcc9dd469ec7525c80c29f0207487d8f26fff34
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>