External snapshot: allow empty snapshot for external snapshot.

src/snapshot/snapshot.h

Index: src/snapshot/snapshot.h
diff --git a/src/snapshot/snapshot.h b/src/snapshot/snapshot.h
index 371afef4fcd5c349f1cbf4f2b280eee692ef78cf..7f89213f16c41c56d881cac7d7ba14ee15a59b2c 100644
--- a/src/snapshot/snapshot.h
+++ b/src/snapshot/snapshot.h
@@ -38,7 +38,8 @@ class Snapshot : public AllStatic {
 
   static bool HaveASnapshotToStartFrom(Isolate* isolate) {
     // Do not use snapshots if the isolate is used to create snapshots.
-    return isolate->snapshot_blob() != NULL;
+    return isolate->snapshot_blob() != NULL &&
+           isolate->snapshot_blob()->data != NULL;
   }
 
   static bool EmbedsScript(Isolate* isolate);