Handle edge cases in feistel permutation

[niklas-uhl]

There might still be happening weird things when the number of vertices to permute is zero, we should rename most_significant_bit_set to most_significant_bit, and it should probably return an optional. But this is something for another PR.

Originally posted by @niklas-uhl in https://github.com/KarlsruheGraphGeneration/KaGen/issues/87#issuecomment-3613577105