[SYCL][Graph] Fix minor issues in graph update code (#13660)

- Check for empty cmd group when updating node reqs
- Fix nullptr deref in reqs check

sycl/source/detail/graph_impl.cpp

@@ -1281,6 +1281,8 @@ void exec_graph_impl::update(
   // Rebuild cached requirements for this graph with updated nodes
   MRequirements.clear();
   for (auto &Node : MNodeStorage) {
+    if (!Node->MCommandGroup)
+      continue;
     MRequirements.insert(MRequirements.end(),
                          Node->MCommandGroup->getRequirements().begin(),
                          Node->MCommandGroup->getRequirements().end());

sycl/source/detail/graph_impl.hpp

@@ -181,6 +181,9 @@ class node_impl {
   /// @param IncomingReq Incoming requirement.
   /// @return True if a dependency is needed, false if not.
   bool hasRequirementDependency(sycl::detail::AccessorImplHost *IncomingReq) {
+    if (!MCommandGroup)
+      return false;
+
     access_mode InMode = IncomingReq->MAccessMode;
     switch (InMode) {
     case access_mode::read: