[SCEV] Fix potentially empty set for unsigned ranges

The following commit enabled the analysis of ranges for heap allocations:
22ca38da

The range turns out to be empty in cases such as the one in test (which
is [1,1)), leading to an assertion failure. This patch fixes for the
same case.

Fixes https://github.com/llvm/llvm-project/issues/63856

Reviewed By: fhahn

Differential Revision: https://reviews.llvm.org/D159160