Fix #8594: autodoc: empty __all__ attribute is ignored

An empty `__all__` should be represented as "there is no public items".
But autodoc considers all items on the module are public.  This changes
the behavior to correct one.

tests/roots/test-ext-autodoc/target/empty_all.py

@@ -0,0 +1,16 @@
+"""
+docsting of empty_all module.
+"""
+__all__ = []
+
+
+def foo():
+    """docstring"""
+
+
+def bar():
+    """docstring"""
+
+
+def baz():
+    """docstring"""