[FIX] web_editor: avoid conflict dialog when no versioning

When the user creates a new article in Knowledge using a template, the
system shows a dialog indicating that there is a conflict between the
client and the server. This commit will ensure that the conflict dialog
won't show up when the document does not have any versioning yet.

Steps to reproduce the issue:

1. Go on Knowledge
2. Open the editor
3. Click on the "Browse Template" button
4. Choose a template
5. Click on the "Load Template" button

Justification
---

When the editor is started, the system generates a new snapshot with ids
generated randomly (see: constructor of `OdooEditor`, `historyReset`,
`_historyGetSnapshotStep`).

When checking if the document is "stale" (i.e: out of sync), the system
verifies the existence of a history step with the id specified in the
`data-last-history-steps` attribute of the first node of the document
(see: `_isLastDocumentStale`). If such a history step does not exist,
the client document is considered "stale" and will be discarded. The
`data-last-history-steps` attribute is set by the last writer when
saving the document.

When creating an article from a template, the server generates a new
record and copies the template's body in the article's body. The system
does not set any `data-last-history-steps` in the document. As a result,
the document will be considered "stale" when opening the article and a
conflict dialog will warn the user that the current document will be
discarded.

This commit will update the `_isLastDocumentStale` function to ensure
that a document that has not been versioned yet won't be considered as
"stale". The `data-last-history-steps` attribute will then be set when
the user opens the article and save it.

task-3587990

closes odoo/odoo#143194

X-original-commit: 69e7694580ae20bb21de1316de5aa770e7ed8eed
Signed-off-by: Nicolas Bayet (nby) <nby@odoo.com>
Signed-off-by: Julien Banken (jbn) <jbn@odoo.com>

addons/web_editor/static/src/js/wysiwyg/wysiwyg.js

@@ -3007,6 +3007,9 @@ export class Wysiwyg extends Component {
         return record;
     }
     _isLastDocumentStale() {
+        if (!this._serverLastStepId) {
+            return false;
+        }
         return !this.odooEditor.historyGetBranchIds().includes(this._serverLastStepId);
     }
     /**