Ensure mute rules cannot be empty

app/models/mute_rule.rb

@@ -1,6 +1,8 @@
 class MuteRule < ApplicationRecord
   belongs_to :user
 
+  validates_presence_of :muted_phrase
+
   def applies_to?(post)
     !!(post.content =~ /\b#{Regexp.escape(muted_phrase)}\b/i)
   end