extend condition to also filter by field exists or not

[dafnapension]

Currently, Filter by condition throws an exception if a field participating in the condition is missing from the instance.
Suggesting here to extend conditions to also handle the very existence of the field in the instance as a measure of this instance being required.
This can be handy for cleaning datasets in some of the cards, which now throw exception for a missing field (breaking the pre-processing pipeline), and also, indirectly, shed light on cards that are totally erroneous, and are better be removed from the catalog:
E.g. for helpful cleaning:
Currently:

whereas pushing the new operators in the card, yields:

Another example, that makes us consider a removal of a totally erroneous card from the catalog:

while with the new filter, we find that the whole subset is missing that field, and we may want to remove the card (for subset 'experts') from the catalog altogether:

Just to prove the point that the whole dataset, of subset experts, does not match the preprocessing steps:

as compared to a subset that does match:

[elronbandel]

Thanks @dafnapension !