Ensure when tag text is None that it is converted to empty string

Fixes Python-Markdown#1049

docs/change_log/index.md

@@ -6,6 +6,7 @@ Python-Markdown Change Log
 Under development: version 3.3.3 (a bug-fix release).
 
 * Unify all block-level tags (#1047).
+* Fix issue where some empty elements would have text rendered as `None` when using `md_in_html` (#1049).
 
 Oct 19, 2020: version 3.3.2 (a bug-fix release).
 

markdown/extensions/md_in_html.py

@@ -260,6 +260,8 @@ def parse_element_content(self, element):
 
         else:
             # Disable inline parsing for everything else
+            if element.text is None:
+                element.text = ''
             element.text = util.AtomicString(element.text)
             for child in list(element):
                 self.parse_element_content(child)

tests/test_syntax/extensions/test_md_in_html.py

@@ -390,6 +390,24 @@ def test_md1_nested_empty_block(self):
             )
         )
 
+    def test_empty_tags(self):
+        self.assertMarkdownRenders(
+            self.dedent(
+                """
+                <div markdown="1">
+                <div></div>
+                </div>
+                """
+            ),
+            self.dedent(
+                """
+                <div>
+                <div></div>
+                </div>
+                """
+            )
+        )
+
     def test_orphan_end_tag_in_raw_html(self):
         self.assertMarkdownRenders(
             self.dedent(