Refactor: libpacemaker: pcmk__create_migration_actions drop redundancy

custom_action()'s return value is guaranteed non-NULL.
pcmk__create_migration_actions() is the only caller that checks the
return value, and even it checks only the migrate_from and migrate_to
values. Drop those unnecessary checks.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>