Improve type annotation of DiscoverRunner. (#1069)

The return type of DiscoverRunner.get_resultclass should
be Optional[Type[TextTestResult]] given that it may also return PDBDebugResult:
https://github.com/django/django/blob/0dd29209091280ccf34e07c9468746c396b7778e/django/test/runner.py#L958-L962

`DicoverRunner.run_tests` accepts `None` for `extra_tests` because
that's already the default value for it:
https://github.com/django/django/blob/0dd29209091280ccf34e07c9468746c396b7778e/django/test/runner.py#L1030

django-stubs/test/runner.pyi

@@ -122,15 +122,15 @@ class DiscoverRunner:
         self, test_labels: Sequence[str] = ..., extra_tests: Optional[List[Any]] = ..., **kwargs: Any
     ) -> TestSuite: ...
     def setup_databases(self, **kwargs: Any) -> List[Tuple[BaseDatabaseWrapper, str, bool]]: ...
-    def get_resultclass(self) -> Optional[Type[DebugSQLTextTestResult]]: ...
+    def get_resultclass(self) -> Optional[Type[TextTestResult]]: ...
     def get_test_runner_kwargs(self) -> Dict[str, Optional[int]]: ...
     def run_checks(self, databases: Set[str]) -> None: ...
     def run_suite(self, suite: TestSuite, **kwargs: Any) -> TextTestResult: ...
     def teardown_databases(self, old_config: List[Tuple[BaseDatabaseWrapper, str, bool]], **kwargs: Any) -> None: ...
     def teardown_test_environment(self, **kwargs: Any) -> None: ...
     def suite_result(self, suite: TestSuite, result: TextTestResult, **kwargs: Any) -> int: ...
     def get_databases(self, suite: TestSuite) -> Set[str]: ...
-    def run_tests(self, test_labels: List[str], extra_tests: List[Any] = ..., **kwargs: Any) -> int: ...
+    def run_tests(self, test_labels: List[str], extra_tests: Optional[List[Any]] = ..., **kwargs: Any) -> int: ...
 
 def is_discoverable(label: str) -> bool: ...
 def reorder_suite(