migrate html_add_permalinks=None and html_add_permalinks=""

The docs list: Set it to None or the empty string to disable permalinks.

sphinx/builders/html/__init__.py

@@ -1223,12 +1223,15 @@ def validate_html_favicon(app: Sphinx, config: Config) -> None:
 
 def migrate_html_add_permalinks(app: Sphinx, config: Config) -> None:
     """Migrate html_add_permalinks to html_permalinks*."""
-    if config.html_add_permalinks:
-        if (isinstance(config.html_add_permalinks, bool) and
-                config.html_add_permalinks is False):
-            config.html_permalinks = False  # type: ignore
-        else:
-            config.html_permalinks_icon = html.escape(config.html_add_permalinks)  # type: ignore  # NOQA
+    html_add_permalinks = config.html_add_permalinks
+    if html_add_permalinks is UNSET:
+        return
+
+    if html_add_permalinks is None or html_add_permalinks is False or html_add_permalinks == "":
+        config.html_permalinks = False  # type: ignore
+        return
+
+    config.html_permalinks_icon = html.escape(config.html_add_permalinks)  # type: ignore  # NOQA
 
 
 # for compatibility
@@ -1237,6 +1240,8 @@ def migrate_html_add_permalinks(app: Sphinx, config: Config) -> None:
 import sphinx.builders.dirhtml  # NOQA
 import sphinx.builders.singlehtml  # NOQA
 
+UNSET = object
+
 
 def setup(app: Sphinx) -> Dict[str, Any]:
     # builders
@@ -1261,7 +1266,7 @@ def setup(app: Sphinx) -> Dict[str, Any]:
     app.add_config_value('html_sidebars', {}, 'html')
     app.add_config_value('html_additional_pages', {}, 'html')
     app.add_config_value('html_domain_indices', True, 'html', [list])
-    app.add_config_value('html_add_permalinks', None, 'html')
+    app.add_config_value('html_add_permalinks', UNSET, 'html')
     app.add_config_value('html_permalinks', True, 'html')
     app.add_config_value('html_permalinks_icon', '¶', 'html')
     app.add_config_value('html_use_index', True, 'html')