Fix clang-tidy readability-implicit-bool-conversion warnings

Use explicit != nullptr comparisons for pointer-to-bool conversions
flagged by clang-tidy.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: facontidavide

src/bt_factory.cpp

@@ -486,7 +486,7 @@ void Tree::remapManifestPointers()
     for(auto& node : subtree->nodes)
     {
       const auto* old_manifest = node->config().manifest;
-      if(old_manifest)
+      if(old_manifest != nullptr)
       {
         auto it = manifests.find(old_manifest->registration_ID);
         if(it != manifests.end())

src/tree_node.cpp

@@ -481,7 +481,7 @@ AnyPtrLocked BT::TreeNode::getLockedPortContent(const std::string& key)
   {
     const auto bb_key = std::string(*remapped_key);
     auto result = _p->config.blackboard->getAnyLocked(bb_key);
-    if(!result && _p->config.manifest)
+    if(!result && _p->config.manifest != nullptr)
     {
       // Entry doesn't exist yet. Create it using the port's type info
       // from the manifest so that getLockedPortContent works even when