Delete any note from the index if noteable is nil

Prior to this we were specifically targetting missing commits because
this was a case we [observed in
staging](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/55878)
but we also may have more cases we haven't run into yet for other
noteable types. The `noteable_id` column on the table does not have a
`NOT NULL` constraint nor does it have foreign key references so it's
totally conceivable there are other kinds of orphaned notes (eg. issues
that were hard deleted at some point) that we don't know about. I have
concerns that any one of these situations could again lead to the
migration to update permissions getting stuck forever. As such I think
the best course of action is to delete any of these. I also can't think
of any good reason to keep the orphaned notes in the index so this seems
quite safe.

ee/lib/elastic/latest/note_instance_proxy.rb

@@ -6,10 +6,11 @@ module Elastic
       delegate :noteable, :noteable_type, to: :target
 
       def as_indexed_json(options = {})
-        # Notes on commits should return the commit object when `notable` is called. However, `noteable` can be null
-        # when a commit has been deleted so an error is raised to alert the caller that the document should be deleted
-        # from the index.
-        raise Elastic::Latest::DocumentShouldBeDeletedFromIndexError.new(target.class.name, target.id) if noteable_type == 'Commit' && noteable.nil?
+        # `noteable` can be sometimes be nil (eg. when a commit has been
+        # deleted) or somehow it was left orphaned in the database. In such
+        # cases we want to delete it from the index since there is no value in
+        # having orphaned notes be searchable.
+        raise Elastic::Latest::DocumentShouldBeDeletedFromIndexError.new(target.class.name, target.id) if noteable.nil?
 
         data = {}
 

ee/spec/models/concerns/elastic/note_spec.rb

@@ -121,8 +121,8 @@ RSpec.describe Note, :elastic do
       expect(note.__elasticsearch__.as_indexed_json).to eq(expected_hash)
     end
 
-    it 'does raises Elastic::Latest::DocumentShouldBeDeletedFromIndexError when commit is not found and noteable is null' do
-      note = create(:note_on_commit)
+    it 'raises Elastic::Latest::DocumentShouldBeDeletedFromIndexError when noteable is nil' do
+      note = create(:note_on_issue)
       allow(note).to receive(:noteable).and_return(nil)
 
       expect { note.__elasticsearch__.as_indexed_json }.to raise_error(::Elastic::Latest::DocumentShouldBeDeletedFromIndexError)