Changed how ParsedInstance objects are converted to mongodb documents: if there is no deleted date attribute, it should not be present in the document at all (this will also simplify the corresponding query).

Conflicts:
	odk_viewer/models/parsed_instance.py
Updated the mongodb queries accordingly: since {"_deleted_at": None} is logically equivalent to "": [{"_deleted_at": {"": False}}, {"_deleted_at": None}] there is no reason to introduce an additional 'or' clause in the query.
The variable: value combination '_deleted_at': None should not be part of the expected_data dict in the pdk_viewer tests.
Another adjustment to the test: if a field is truly optional/ignorable, it may not be present in a given dict, so don't pop it off w/o checking first.
More tests expected {'_deleted_at': None} -- at least they were consistent.
Why does '_deleted_at' exist in AbstractDataFrameBuilder.IGNORED_COLUMNS and then is *required* in a test?