Raise TypeError for with raises(..., match=<non-None falsey value>).

src/_pytest/python_api.py

@@ -716,6 +716,6 @@ class RaisesContext(object):
         suppress_exception = issubclass(self.excinfo.type, self.expected_exception)
         if sys.version_info[0] == 2 and suppress_exception:
             sys.exc_clear()
-        if self.match_expr and suppress_exception:
+        if self.match_expr is not None and suppress_exception:
             self.excinfo.match(self.match_expr)
         return suppress_exception