fix(ngOptions): select unknown option if unmatched model does not match empty option

When a regular / ngOptions select has an explicit *empty* option, this option can be selected
by the user and will set the model to `null`. It is also selected when the model is set to
`null` or `undefined`.

When the model is set to a value that does not match any option value, and is also not
`null` or `undefined`, the *unknown* option is inserted and selected - this is an explicit marker
that the select is in an invalid / unknown state, which is different from an allowed empty state.

Previously, regular selects followed this logic, whereas ngOptions selects selected the empty
option in the case described above.

This patch makes the behavior consistent between regular / ngOptions select - the latter will now
insert and select the unknown option. The order of the options has been fixed to unknown -> empty
-> actual options.