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/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

src/xml_parsing.cpp

@@ -870,7 +870,7 @@ TreeNode::Ptr XMLParser::PImpl::createNodeFromXML(const XMLElement* element,
     // If a substitution rule replaced the SubTree with a different node
     // (e.g. a TestNode), the dynamic_cast will return nullptr.
     auto subtree_node = dynamic_cast<SubTreeNode*>(new_node.get());
-    if(subtree_node)
+    if(subtree_node != nullptr)
     {
       subtree_node->setSubtreeID(type_ID);
     }