[Assignment Tracking] Skip empty-metadata dbg.declares in AssignmentTrackingPass

Debug intrinsics sometimes end up with empty metadata location operands. The
debug intrinsic interfaces return nullptr when retrieving location operand in
this case.

Skip empty-metadata dbg.declares to avoid dereferencing the nullptr. This
doesn't affect the final debug info in any way.

Reviewed By: jryans

Differential Revision: https://reviews.llvm.org/D148204