webadmin,userportal: Fix null-command grid button visibility

Grid buttons whose UICommand is null fall back to using empty
(no-op) command singleton object (EMPTY_COMMAND).

Previously, EMPTY_COMMAND was declared with IsAvailable and
IsExecutionAllowed properties both set to 'false'. This meant
that the corresponding UI button was both hidden and disabled.

After this patch, EMPTY_COMMAND leaves IsAvailable property
to its original value ('true'), so that the corresponding UI
button stays visible at all times.

Change-Id: Ic04e99901e59bb4fb85afac1e8db22088ecb3f59
Signed-off-by: Vojtech Szocs <vszocs@redhat.com>