[TEST]  NonEmptyTest.testNonEmptyCJWithMultiPositionSlicer was failing f...

...or two reasons:

1)  During native evaluation, attempts to make a cell request with the "placeholder" compound slicer member in context were failing, since the #null member is used as the placeholder, and the level associated w/ the null member does not have a level reader.  Also, the #null member would always be treated as needing to return null.  To address this, swapped the placeholder to be (All).

2)  The tuple list is cached with the CompoundSlicerRolapMember as part of the key, but CSRM did not implement equals or hashcode.

This does not impact Mondrian 3, since Compound slicers are handled directly in native evaluation (MONDRIAN-1420).