Fix link previews (part 2) (#3073)

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### What

### Checklist
* [x] I have read and agree to [Contributor
Guide](https://github.com/rerun-io/rerun/blob/main/CONTRIBUTING.md) and
the [Code of
Conduct](https://github.com/rerun-io/rerun/blob/main/CODE_OF_CONDUCT.md)
* [x] I've included a screenshot or gif (if applicable)
* [x] I have tested [demo.rerun.io](https://demo.rerun.io/pr/3073) (if
applicable)

- [PR Build Summary](https://build.rerun.io/pr/3073)
- [Docs
preview](https://rerun.io/preview/e3f65bfab3f8192853f1942fc070c5c3689fb464/docs)
<!--DOCS-PREVIEW-->
- [Examples
preview](https://rerun.io/preview/e3f65bfab3f8192853f1942fc070c5c3689fb464/examples)
<!--EXAMPLES-PREVIEW--><!--EXAMPLES-PREVIEW--><!--EXAMPLES-PREVIEW-->
- [Recent benchmark results](https://ref.rerun.io/dev/bench/)
- [Wasm size tracking](https://ref.rerun.io/dev/sizes/)

scripts/ci/update_pr_body.py

@@ -54,15 +54,19 @@ def main() -> None:
 
     new_body = pr.body
 
-    docs_preview_link_end = new_body.find(DOCS_PREVIEW_MARKER) + len(DOCS_PREVIEW_MARKER)
-    docs_preview_link_start = new_body.rfind("\n", 0, docs_preview_link_end) + 1
-    new_body = new_body[:docs_preview_link_start] + DOCS_PREVIEW_BARE_LINK + new_body[docs_preview_link_end:]
-
-    examples_preview_link_end = new_body.find(EXAMPLES_PREVIEW_MARKER) + len(EXAMPLES_PREVIEW_MARKER)
-    examples_preview_link_start = new_body.rfind("\n", 0, examples_preview_link_end) + 1
-    new_body = (
-        new_body[:examples_preview_link_start] + EXAMPLES_PREVIEW_BARE_LINK + new_body[examples_preview_link_end:]
-    )
+    docs_preview_link_end = new_body.find(DOCS_PREVIEW_MARKER)
+    if docs_preview_link_end != -1:
+        docs_preview_link_end += len(DOCS_PREVIEW_MARKER)
+        docs_preview_link_start = new_body.rfind("\n", 0, docs_preview_link_end) + 1
+        new_body = new_body[:docs_preview_link_start] + DOCS_PREVIEW_BARE_LINK + new_body[docs_preview_link_end:]
+
+    examples_preview_link_end = new_body.find(EXAMPLES_PREVIEW_MARKER)
+    if examples_preview_link_end != -1:
+        len(EXAMPLES_PREVIEW_MARKER)
+        examples_preview_link_start = new_body.rfind("\n", 0, examples_preview_link_end) + 1
+        new_body = (
+            new_body[:examples_preview_link_start] + EXAMPLES_PREVIEW_BARE_LINK + new_body[examples_preview_link_end:]
+        )
 
     new_body = env.from_string(new_body).render(
         pr={