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/init.cpp

@@ -1807,7 +1807,7 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
     {
         WAIT_LOCK(kernel_notifications.m_tip_block_mutex, lock);
         kernel_notifications.m_tip_block_cv.wait(lock, [&]() EXCLUSIVE_LOCKS_REQUIRED(kernel_notifications.m_tip_block_mutex) {
-            return kernel_notifications.m_tip_block || ShutdownRequested(node);
+            return kernel_notifications.TipBlock() || ShutdownRequested(node);
         });
     }