How to check consecutive number in list?

By mathematical convention as discussed in the comments, it would be standard to consider both an empty list and a list with a single element as consecutive, should it not be specified in your homework more precisely.

By the logic of vacuous truth, were a list to not have consecutive order it would require enough elements (2) to break this condition of ordering. To handle this in your existing code, you could simply perform a check before your main check to ensure the base cases of an empty list and a list with one element return True.