[ruby/rdoc] fix: alias to method with call-seq

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

1 files changed, 1 insertions, 1 deletions

diff --git a/lib/rdoc/any_method.rb b/lib/rdoc/any_method.rb
index 3d0b60790d..0b7dd717ab 100644
--- a/lib/rdoc/any_method.rb
+++ b/lib/rdoc/any_method.rb
@@ -359,6 +359,6 @@ class RDoc::AnyMethod < RDoc::MethodAttr
         entry =~ /\s#{ignore}\s/
     end
 
-    matching.join "\n"
+    matching.empty? ? nil : matching.join("\n")
   end
 end