Don't assert when ServiceWorker::from is passed an empty promise

Source/modules/serviceworkers/ServiceWorker.cpp

Index: Source/modules/serviceworkers/ServiceWorker.cpp
diff --git a/Source/modules/serviceworkers/ServiceWorker.cpp b/Source/modules/serviceworkers/ServiceWorker.cpp
index 84bca65e79e81427d9d0f4b574245993cbde3738..1e3163b9e171f5993ed22fba43f81e61c907bda5 100644
--- a/Source/modules/serviceworkers/ServiceWorker.cpp
+++ b/Source/modules/serviceworkers/ServiceWorker.cpp
@@ -195,6 +195,12 @@ void ServiceWorker::onPromiseResolved()
 
 void ServiceWorker::waitOnPromise(ScriptPromise promise)
 {
+    if (promise.isEmpty()) {
+        // The document was detached during registration. The state doesn't really
+        // matter since this ServiceWorker will immediately die.
+        setProxyState(ContextStopped);
+        return;
+    }
     setProxyState(RegisterPromisePending);
     promise.then(ThenFunction::create(this));
 }