Empty embeds when RichText is set to an empty content

Fix rails#51269

Action Text supports saving a RichText model with empty content. Passing _almost_ empty content (such as a space) flushes the embeds, but passing truly empty content does not. This defies expectations.

This change ensures that the embeds are flushed when empty content is passed.

actiontext/app/models/action_text/rich_text.rb

@@ -52,7 +52,7 @@ class RichText < Record
     has_many_attached :embeds
 
     before_save do
-      self.embeds = body.attachables.grep(ActiveStorage::Blob).uniq if body.present?
+      self.embeds = body.nil? ? [] : body.attachables.grep(ActiveStorage::Blob).uniq
     end
 
     # Returns a plain-text version of the markup contained by the `body` attribute,

actiontext/test/unit/model_test.rb

@@ -57,6 +57,28 @@ class ActionText::ModelTest < ActiveSupport::TestCase
     end
   end
 
+  test "saving empty content empties the embeds" do
+    blob = create_file_blob(filename: "racecar.jpg", content_type: "image/jpeg")
+    message = Message.create!(subject: "Greetings", content: ActionText::Content.new("Hello world").append_attachables(blob))
+    assert message.content.embeds.sole
+
+    message.content = ""
+    message.save
+
+    assert_empty message.content.embeds
+  end
+
+  test "saving nil content empties the embeds" do
+    blob = create_file_blob(filename: "racecar.jpg", content_type: "image/jpeg")
+    message = Message.create!(subject: "Greetings", content: ActionText::Content.new("Hello world").append_attachables(blob))
+    assert message.content.embeds.sole
+
+    message.content = nil
+    message.save
+
+    assert_empty message.content.embeds
+  end
+
   test "saving content" do
     message = Message.create!(subject: "Greetings", content: "<h1>Hello world</h1>")
     assert_equal "Hello world", message.content.to_plain_text