Fix #7461: autodoc: empty tuple in type annotation is not shown correctly

CHANGES

@@ -17,6 +17,7 @@ Bugs fixed
 ----------
 
 * #7461: py domain: fails with IndexError for empty tuple in type annotation
+* #7461: autodoc: empty tuple in type annotation is not shown correctly
 
 Testing
 --------

sphinx/pycode/ast.py

@@ -114,7 +114,10 @@ def unparse(node: ast.AST) -> str:
     elif isinstance(node, ast.UnaryOp):
         return "%s %s" % (unparse(node.op), unparse(node.operand))
     elif isinstance(node, ast.Tuple):
-        return ", ".join(unparse(e) for e in node.elts)
+        if node.elts:
+            return ", ".join(unparse(e) for e in node.elts)
+        else:
+            return "()"
     elif sys.version_info > (3, 6) and isinstance(node, ast.Constant):
         # this branch should be placed at last
         return repr(node.value)

tests/test_pycode_ast.py

@@ -54,6 +54,7 @@ from sphinx.pycode import ast
     ("- 1", "- 1"),                             # UnaryOp
     ("- a", "- a"),                             # USub
     ("(1, 2, 3)", "1, 2, 3"),                   # Tuple
+    ("()", "()"),                               # Tuple (empty)
 ])
 def test_unparse(source, expected):
     module = ast.parse(source)