Fixed Python 3.12 DeprecationWarning (#2055)

Testing an element's truth value will raise an exception in future versions.

pymdownx/details.py

@@ -85,7 +85,7 @@ def parse_content(self, parent, block):
             # to be under it, then the content's is sibling is in the list.
             last_child = self.lastChild(sibling)
             indent = 0
-            while last_child:
+            while last_child is not None:
                 if (
                     sibling and block.startswith(' ' * self.tab_length * 2) and
                     last_child and last_child.tag in ('ul', 'ol', 'dl')