Fix error with filenames ending with a dot (#3349)

The second negative argument to split instructs it to
preserve null fields in the returned array

lib/linguist/blob.rb

@@ -63,7 +63,7 @@ module Linguist
     #
     # Returns an Array
     def extensions
-      _, *segments = name.downcase.split(".")
+      _, *segments = name.downcase.split(".", -1)
 
       segments.map.with_index do |segment, index|
         "." + segments[index..-1].join(".")