pathlib ABCs: support initializing paths with no arguments (#126608)

In the past I've equivocated about whether to require at least one argument
in the `PurePathBase` (and `PathBase`) initializer, and what the default
should be if we make it optional. I now have a local use case that has
persuaded me to make it optional and default to the empty string (a
`zipp.Path`-like class that treats relative and absolute paths similarly.)
Happily this brings the base class more in line with `PurePath` and `Path`.

Lib/test/test_pathlib/test_pathlib_abc.py

@@ -148,6 +148,7 @@ class DummyPurePathTest(unittest.TestCase):
         P = self.cls
         p = P('a')
         self.assertIsInstance(p, P)
+        P()
         P('a', 'b', 'c')
         P('/a', 'b', 'c')
         P('a/b/c')