Don't blow up if empty string/nil passed to alias methods

lib/linguist/language.rb

@@ -150,6 +150,7 @@ module Linguist
     #
     # Returns the Language or nil if none was found.
     def self.find_by_name(name)
+      return nil if name.to_s.empty?
       name && (@name_index[name.downcase] || @name_index[name.split(',').first.downcase])
     end
 
@@ -164,6 +165,7 @@ module Linguist
     #
     # Returns the Language or nil if none was found.
     def self.find_by_alias(name)
+      return nil if name.to_s.empty?
       name && (@alias_index[name.downcase] || @alias_index[name.split(',').first.downcase])
     end