Remove "Empty filter" type in favor of no filter row

[pmeier]

Reported in #499 (comment)

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Instead of having an "Empty Filter" type that indicates that we don't want to apply any filter, we should just have no filter row at all.

ragna/ragna/deploy/_ui/components/metadata_filters_builder.py

Lines 95 to 96 in 9221bb8

	if self.key_select.value == NO_FILTER_KEY:
	return None

See #499 (comment) for a non-exhaustive list of problems that we'll have to solve otherwise.

Ultimately, we need to adapt the code such that we don't need the if filter_row.key != NO_FILTER_KEY in

ragna/ragna/deploy/_ui/components/metadata_filters_builder.py

Lines 198 to 207 in 9221bb8

	def construct_metadata_filters(self):
	metadata_filters = [
	filter_row.construct_metadata_filter()
	for filter_row in self.filter_rows
	if filter_row.key != NO_FILTER_KEY
	]
	if not metadata_filters:
	return None
	return MetadataFilter.and_(metadata_filters).to_primitive()