KeywordMute matcher: more closely mimic Regexp#=~ behavior.

Regexp#=~ returns nil if it does not match.  An empty mute set does not
match any status, so KeywordMute::Matcher#=~ ought to return nil also.

app/models/glitch/keyword_mute.rb

@@ -61,7 +61,7 @@ class Glitch::KeywordMute < ApplicationRecord
     end
 
     def =~(str)
-      regex ? regex =~ str : false
+      regex ? regex =~ str : nil
     end
   end
 end