Fix bad_version when m_notifiers is empty (#424)

* Fix bad_version when m_notifiers is empty

realm/realm-swift#4768
realm/realm-java#4369

When m_notifiers is empty, m_notifier_sg will be advanced to the latest.
But the skip_version might be behind. In this case, bad_version will be
throw when calling advance_to_final().

* Add a test which reproduces the problem

src/impl/realm_coordinator.cpp

@@ -496,6 +496,7 @@ void RealmCoordinator::clean_up_dead_notifiers()
         if (m_notifiers.empty() && m_notifier_sg) {
             REALM_ASSERT_3(m_notifier_sg->get_transact_stage(), ==, SharedGroup::transact_Reading);
             m_notifier_sg->end_read();
+            m_notifier_skip_version = {0, 0};
         }
     }
     if (swap_remove(m_new_notifiers) && m_advancer_sg) {
@@ -693,6 +694,7 @@ void RealmCoordinator::run_async_notifiers()
     lock.unlock();
 
     if (skip_version.version) {
+        REALM_ASSERT(!notifiers.empty());
         REALM_ASSERT(version >= skip_version);
         IncrementalChangeInfo change_info(*m_notifier_sg, notifiers);
         for (auto& notifier : notifiers)

tests/results.cpp

@@ -834,6 +834,27 @@ TEST_CASE("notifications: skip") {
         advance_and_notify(*r);
         REQUIRE(calls1 == 2);
     }
+
+    SECTION("removing skipped notifier before it gets the chance to run") {
+        advance_and_notify(*r);
+        REQUIRE(calls1 == 1);
+
+        // Set the skip version
+        make_local_change(token1);
+        // Advance the file to a version after the skip version
+        make_remote_change();
+        REQUIRE(calls1 == 1);
+
+        // Remove the skipped notifier and add an entirely new notifier, so that
+        // notifications need to run but the skip logic shouldn't be used
+        token1 = {};
+        results = {};
+        Results results2(r, table->where());
+        auto token2 = add_callback(results2, calls1, changes1);
+
+        advance_and_notify(*r);
+        REQUIRE(calls1 == 2);
+    }
 }
 
 #if REALM_PLATFORM_APPLE