StatusMessage#raw_message returns empty string for NULL values

Previously the federation layer has written the empty string to the
database, now it writes nil/no value to it while our code still expects
the empty string in some cases. Restore the old assumption by returning
the empty string, the real raw value by now is available again in
StatusMessage#text for those that need it, such as validations or tests.

app/models/status_message.rb

@@ -53,7 +53,7 @@ class StatusMessage < Post
   end
 
   def raw_message
-    read_attribute(:text)
+    read_attribute(:text) || ""
   end
 
   def raw_message=(text)

spec/models/status_message_spec.rb

@@ -113,6 +113,9 @@ describe StatusMessage, type: :model do
 
       post.photos << photo
       expect(post).to be_valid
+      expect(post.message.to_s).to be_empty
+      expect(post.raw_message).to eq ""
+      expect(post.nsfw).to be_falsy
       expect(post.errors.full_messages).to eq([])
     end