Change default for redirect param of create() and login() to None

Fix the following mypy error:

```
lib/tool_shed/test/functional/test_frontend_login.py:76: error: Argument
"redirect" to "login" of "ShedTwillTestCase" has incompatible type "None";
expected "str"  [arg-type]
            self.login(email, TEST_PASSWORD, username=user, redirect=None)
                                                                     ^
```

lib/tool_shed/test/base/twilltestcase.py

@@ -15,6 +15,7 @@
     Iterator,
     List,
     Optional,
+    Tuple,
 )
 from urllib.parse import (
     quote_plus,
@@ -694,7 +695,14 @@ def _submit_register_form(self, email: str, password: str, username: str, redire
     def invalid_tools_labels(self) -> str:
         return "Invalid Tools" if self.is_v2 else "Invalid tools"
 
-    def create(self, cntrller="user", email="[email protected]", password="testuser", username="admin-user", redirect=""):
+    def create(
+        self,
+        cntrller: str = "user",
+        email: str = "[email protected]",
+        password: str = "testuser",
+        username: str = "admin-user",
+        redirect: Optional[str] = None,
+    ) -> Tuple[bool, bool, bool]:
         # HACK: don't use panels because late_javascripts() messes up the twill browser and it
         # can't find form fields (and hence user can't be logged in).
         params = dict(cntrller=cntrller, use_panels=False)
@@ -747,7 +755,7 @@ def login(
         email: str = "[email protected]",
         password: str = "testuser",
         username: str = "admin-user",
-        redirect: str = "",
+        redirect: Optional[str] = None,
         logout_first: bool = True,
     ):
         if self.is_v2:

lib/tool_shed/test/functional/test_frontend_login.py

@@ -73,4 +73,4 @@ def _create_and_login(self):
             password=TEST_PASSWORD,
             username=user,
         )
-        self.login(email, TEST_PASSWORD, username=user, redirect=None)
+        self.login(email, TEST_PASSWORD, username=user)