[Password Manager] Check for null database before migrating

When PasswordStoreDefault initialises the LoginDatabase, if the step fails,
it will set the database to null. In that case, PasswordStoreX should not
attempt to perform a migration.

Bug: 991213
Change-Id: Id31a65b228532c1206283f6af175f339efbb292b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1746167Reviewed-by: Vadym Doroshenko <dvadym@chromium.org>
Commit-Queue: Christos Froussios <cfroussios@chromium.org>
Cr-Commit-Position: refs/heads/master@{#686430}

chrome/browser/password_manager/password_store_x.cc

@@ -207,12 +207,18 @@ void PasswordStoreX::CheckMigration() {
 
   if (migration_checked_)
     return;
-
   migration_checked_ = true;
+
   if (migration_to_login_db_step_ == LOGIN_DB_REPLACED) {
     return;
   }
 
+  if (!login_db()) {
+    LOG(ERROR) << "Could not start the migration into the encrypted "
+                  "LoginDatabase because the database failed to initialise.";
+    return;
+  }
+
   // If the db is empty, there are no records to migrate, and we then can call
   // it a completed migration.
   if (login_db()->IsEmpty()) {