bpo-35742: Fix test_envar_unimportable in test_builtin. (GH-11561)

Handle the case of an empty module name in PYTHONBREAKPOINT.

Fixes a regression introduced in bpo-34756.

Lib/test/test_builtin.py

@@ -1608,6 +1608,7 @@ class TestBreakpoint(unittest.TestCase):
     def test_envar_unimportable(self):
         for envar in (
                 '.', '..', '.foo', 'foo.', '.int', 'int.',
+                '.foo.bar', '..foo.bar', '/./',
                 'nosuchbuiltin',
                 'nosuchmodule.nosuchcallable',
                 ):