webdriver: read_global returns None when getActiveElement test dismisess alert (#8480)

Using the user prompt handler to automatically dismiss an alert should
result in a value of False, not None for the retrieved JavaScript variable
on the page.
diff --git a/webdriver/tests/element_retrieval/get_active_element.py b/webdriver/tests/element_retrieval/get_active_element.py
index 5c2b52a..af3e125 100644
--- a/webdriver/tests/element_retrieval/get_active_element.py
+++ b/webdriver/tests/element_retrieval/get_active_element.py
@@ -69,7 +69,7 @@
     response = get_active_element(session)
     assert_is_active_element(session, response)
     assert_dialog_handled(session, "dismiss #2")
-    assert read_global(session, "dismiss2") is None
+    assert read_global(session, "dismiss2") is False
 
     create_dialog(session)("prompt", text="dismiss #3", result_var="dismiss3")