Ensure m_tip_block is never ZERO

To avoid future code changes from reintroducing the ambiguity fixed
by the previous commit, mark m_tip_block private and Assume that
it's not set to uint256::ZERO.

src/node/kernel_notifications.cpp

@@ -52,6 +52,7 @@ kernel::InterruptResult KernelNotifications::blockTip(SynchronizationState state
 {
     {
         LOCK(m_tip_block_mutex);
+        Assume(index.GetBlockHash() != uint256::ZERO);
         m_tip_block = index.GetBlockHash();
         m_tip_block_cv.notify_all();
     }
@@ -99,6 +100,13 @@ void KernelNotifications::fatalError(const bilingual_str& message)
                     m_exit_status, message, &m_warnings);
 }
 
+std::optional<uint256> KernelNotifications::TipBlock()
+{
+    AssertLockHeld(m_tip_block_mutex);
+    return m_tip_block;
+};
+
+
 void ReadNotificationArgs(const ArgsManager& args, KernelNotifications& notifications)
 {
     if (auto value{args.GetIntArg("-stopatheight")}) notifications.m_stop_at_height = *value;