This change fixes alias call-seq to return nil if the method's
call-seq does not specify the alias.
Previously, the alias's call-seq would be an empty string in this case
which broke darkfish rendering.
This change also backfills test coverage for 0ead786 which moved
call-seq deduplication into AnyMethod.
https://github.com/ruby/rdoc/commit/5ce2789b6f