[MLIR][Presburger][Simplex] moveRowUnknownToColumn: support the row sample value being zero

When the sample value is zero, everything is the same except that failure to
pivot does not imply emptiness. So, leave it to the user to mark as empty if
necessary, if they know the sample value is strictly negative. This is needed
for an upcoming symbolic lexmin heuristic.

Reviewed By: Groverkss

Differential Revision: https://reviews.llvm.org/D123604