fix: Handle missing or null 'items' key in DataForSEO Related Keywords block (#10989)

### Changes 🏗️

- Modified the DataForSEO Related Keywords block to handle cases where
the 'items' key is missing or has a null value in the API response.
- Ensures that the code gracefully handles these scenarios by defaulting
to an empty list, preventing potential errors. Fixes
[AUTOGPT-SERVER-66D](https://sentry.io/organizations/significant-gravitas/issues/6902944636/).

### Checklist 📋

#### For code changes:
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [x] I have tested my changes according to the test plan:
  <!-- Put your test plan here: -->
- [x] The DataForSEO API now returns an empty list when there are no
results, preventing the code from attempting to iterate on a null value.

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Strengthens parsing of DataForSEO Labs response to avoid errors when
`items` is missing or null.
> 
> - In `backend/blocks/dataforseo/related_keywords.py` `run()`, sets
`items = first_result.get("items") or []` when `first_result` is a
`dict`, otherwise `[]`, ensuring safe iteration
> - Prevents exceptions and yields empty results when no items are
returned
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
cc465ddbf2. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Co-authored-by: seer-by-sentry[bot] <157164994+seer-by-sentry[bot]@users.noreply.github.com>
Co-authored-by: Toran Bruce Richards <toran.richards@gmail.com>
Co-authored-by: claude[bot] <41898282+claude[bot]@users.noreply.github.com>
Co-authored-by: Nicholas Tindle <ntindle@users.noreply.github.com>
Co-authored-by: Nicholas Tindle <nicholas.tindle@agpt.co>

autogpt_platform/backend/backend/blocks/dataforseo/related_keywords.py

@@ -182,13 +182,10 @@ class DataForSeoRelatedKeywordsBlock(Block):
             if results and len(results) > 0:
                 # results is a list, get the first element
                 first_result = results[0] if isinstance(results, list) else results
-                items = (
-                    first_result.get("items", [])
-                    if isinstance(first_result, dict)
-                    else []
-                )
-                # Ensure items is never None
-                if items is None:
+                # Handle missing key, null value, or valid list value
+                if isinstance(first_result, dict):
+                    items = first_result.get("items") or []
+                else:
                     items = []
                 for item in items:
                     # Extract keyword_data from the item