Ensure we don't create gap epochs when importing multiple genesis epochs (#10019)

The babe `authoring_blocks` test was flaky because it could happen that
we imported the first block multiple times. This lead to import the
genesis epoch multiples times. `EpochChanges` was assuming that
importing a genesis epoch while there was already an imported epoch
means that there is a "gap". However, this wasn't true as we just
imported 2 genesis epochs. The bug is solved by checking that the
already imported epochs are not all genesis epochs.