fix warns: prune {} vals from join attr before search_rs()

Fixed long-time unitialized warnings in ResultSet -- tracked down to
being caused by join attrs with empty hash ({}) values which are
generated from the merge_join process. These are now cleaned prior to
$Rs->search_rs(...) calls in DbicLink2.

For reference, this fixes warnings like the following (seen as of
at least DBIx::Class 0.082810) when these join attrs are present:

 Use of uninitialized value $a_key in string eq at lib/DBIx/Class/ResultSet.pm line 3821.
 Use of uninitialized value $b_key in string eq at lib/DBIx/Class/ResultSet.pm line 3821.
 Use of uninitialized value $a_key in hash element at lib/DBIx/Class/ResultSet.pm line 3822.
 Use of uninitialized value $b_key in hash element at lib/DBIx/Class/ResultSet.pm line 3822.
 Use of uninitialized value within @_ in list assignment at lib/DBIx/Class/ResultSet.pm line 3808.
 Use of uninitialized value within @_ in list assignment at lib/DBIx/Class/ResultSet.pm line 3808.
 Odd number of elements in anonymous hash at lib/DBIx/Class/ResultSet.pm line 3872.