Closes #9418: Fix missing bracket for empty Callable annotations.

CHANGES

@@ -16,6 +16,9 @@ Features added
 Bugs fixed
 ----------
 
+* #9418: a Callable annotation with no parameters (e.g. Callable[[], None]) will
+  be rendered with a bracket missing (Callable[], None])
+
 Testing
 --------
 

sphinx/domains/python.py

@@ -129,10 +129,14 @@ def unparse(node: ast.AST) -> List[Node]:
             return unparse(node.value)
         elif isinstance(node, ast.List):
             result = [addnodes.desc_sig_punctuation('', '[')]
-            for elem in node.elts:
-                result.extend(unparse(elem))
-                result.append(addnodes.desc_sig_punctuation('', ', '))
-            result.pop()
+            if node.elts:
+                # check if there are elements in node.elts to only pop the
+                # last element of result if the for-loop was run at least
+                # once
+                for elem in node.elts:
+                    result.extend(unparse(elem))
+                    result.append(addnodes.desc_sig_punctuation('', ', '))
+                result.pop()
             result.append(addnodes.desc_sig_punctuation('', ']'))
             return result
         elif isinstance(node, ast.Module):