Fix some errors that last kiwi change got

Kiwi's proxy widget set _valid = None on __init__, so on some circunstantes
(like when no one called validate, or called it when the widget was
insensitive/invisible) it could return None when calling is_valid, and None is
considered False when tested in a boolean context (and that's what the dumper
was testing to put 'invalid' on the dump).

That make some tests break, like:
* editor-cash*: The dump was saying that the 'reason' was invalid, when
  it really wasn't, because of the problem mentioned above (and there wasn't
  any validator neither mandatory flag set). But it should actually be mandatory.
* search-parameter*: The same problem, but for obvious reasons, the filter
  entry is not invalid when empty or with whatever string you type in it.

Change-Id: I20852ee989f4198df1526c0ab1f6a9feec6cfa78