FIX: Allow user actions to be saved even if the post has nil user

This issue made it impossible to delete users if they had flagged a post with nil user

app/models/post_action.rb

@@ -575,11 +575,11 @@ class PostAction < ActiveRecord::Base
 
   def self.auto_hide_if_needed(acting_user, post, post_action_type)
     return if post.hidden?
-    return if (!acting_user.staff?) && post.user.staff?
+    return if (!acting_user.staff?) && post.user&.staff?
 
     if post_action_type == :spam &&
        acting_user.has_trust_level?(TrustLevel[3]) &&
-       post.user.trust_level == TrustLevel[0]
+       post.user&.trust_level == TrustLevel[0]
 
       hide_post!(post, post_action_type, Post.hidden_reasons[:flagged_by_tl3_user])
 

spec/models/post_action_spec.rb

@@ -477,6 +477,15 @@ describe PostAction do
       expect(post.topic.visible).to eq(false)
     end
 
+    it "doesn't fail when post has nil user" do
+      post = create_post
+      post.update!(user: nil)
+
+      PostAction.act(codinghorror, post, PostActionType.types[:spam], take_action: true)
+      post.reload
+      expect(post.hidden).to eq(true)
+    end
+
     it "hide tl0 posts that are flagged as spam by a tl3 user" do
       newuser = Fabricate(:newuser)
       post = create_post(user: newuser)