Modify User Attributes API - Fix

This fix does not allow empty host strings in the update user call.
Besides, it also enusres that database access is not lost during an update of the host attribute

Change-Id: I57b7733c264d2bcd70dfea95f85aae68553c4dff
Fixes: bug #1214166
Fixes: bug #1214555
Merge "Modify User Attributes API - Fix"