Rename Query._mapper_zero() to Query._entity_zero()

To be more descriptive of the use of _mapper_zero(), rename
it to _entity_zero(), but also supply a new _mapper_zero() function
that more strictly returns a mapper.  The existing
_entity_zero() function is renamed to _query_entity_zero.
_only_mapper_zero() is removed as it isn't used.  Divide up the
existing calling functions to refer to the appropriate new method.

Change-Id: I8780c3235e87b4936c6daf64d9d299b22b6e1260
Fixes: #3608

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