Fix Jinja2 page null guard in mkdocs theme override templates and docs

Co-authored-by: timvink <5570380+timvink@users.noreply.github.com>

Author: Copilot

docs/howto/override-a-theme.md

@@ -32,10 +32,10 @@ Then you can extend the base `mkdocs` theme by adding a new file `docs/overrides
     {{ page.content }}
 
     <!-- This section adds support for localized revision dates -->
-    {% if page.meta.git_revision_date_localized %}
+    {% if page and page.meta.git_revision_date_localized %}
         <small>Last update: {{ page.meta.git_revision_date_localized }}</small>
     {% endif %}
-    {% if page.meta.git_created_date_localized %}
+    {% if page and page.meta.git_created_date_localized %}
         <small>Created: {{ page.meta.git_created_date_localized }}</small>
     {% endif %}
     ```

tests/fixtures/basic_project/docs/overrides_mkdocs_theme/content.html

@@ -11,6 +11,6 @@
 
 {{ page.content }}
 
-{% if page.meta.git_revision_date_localized %}
+{% if page and page.meta.git_revision_date_localized %}
     <small>Last update: {{ page.meta.git_revision_date_localized }}</small>
 {% endif %}