Add _merge_joinpref_attr test w/ {}/undef (empty) vals

Added a test for the case of join attrs specified with empty ({} or
undef) vals (i.e. join => { rel => { rel2 => {} } }). These tests pass,
however, the empty {} vals currently produce uninitialized warnings.

For further reference, these warnings were bypassed for RapidApp in this
commit:

 https://github.com/vanstyn/RapidApp/commit/6f41f6e48

This test was added per the request of @ribasushi