Skip slot lenience on first block in BABE

The genesis header doesn't have the BABE pre-digest and we insert `0` as
slot number. The slot lenience calculation will return the maximum in
this situation. Besides returning the maximum which is not bad at all,
it also prints some a debug message that can be confusing in the first
moment. To prevent printing this debug message, we now just return early
when we see that the parent block is the genesis block.