Quoted from Belen in the bug 2104, we need to implement that for better user experience.

Hi there,

I just checked the email I sent after our meeting (I've attached it) and I
might have understood completely wrong, but I think what we agreed at OSTS was
that we would go away and look into how the combos should behave and come back
to you with a solution, considering that:

1. there should be an 'empty option' (i.e. no value assigned to it) in the
combo box because no default selected value was possible. 

2. the 'empty option' should only appear on loading the combo box, but it
should not be available for selection once another value has been selected
(e.g. once I've selected a machine I can only select a different machine: I
cannot select the 'empty option').

We have run the empty (as in no string) approach past a few people, and
everybody agrees it looks non-standard and, what's worse, is perceived as an
error. From a design point of view, we firmly believe we should not have an
empty combo. If the development team thinks we are in no position to fix this
now, we'll accept that of course: we can always fix it sometime in the future. 

Cheers

Belen