Fix transform node null store cache on exception and restore NUMPY_ARRAY_CONVERTER null check

Two fixes for py:vectypes:

1. declared_transform.hpp: When stores_.insert() creates a new entry with a
   default (null) value and the user callback throws, the null entry remained
   in the cache. On subsequent messages with the same hash, the null store was
   reused and sent downstream, causing null pointer propagation through the
   graph. Fix: wrap the callback in try/catch and erase the entry on exception.

2. modulewrap.cpp: Restore the null check for pyobj in NUMPY_ARRAY_CONVERTER
   that was removed in PR #213. Without it, PyArray_Check(nullptr) and
   Py_DECREF(nullptr) cause undefined behavior, leading to intermittent
   SEGFAULTs or incorrect behavior.

Co-authored-by: greenc-FNAL <2372949+greenc-FNAL@users.noreply.github.com>

Author: Copilot

phlex/core/declared_transform.hpp

@@ -98,18 +98,23 @@ namespace phlex::experimental {
                    } else {
                      accessor a;
                      if (stores_.insert(a, store->index()->hash())) {
-                       auto result = call(ft, messages, std::make_index_sequence<N>{});
-                       ++calls_;
-                       ++product_count_[store->index()->layer_hash()];
-                       products new_products;
-                       new_products.add_all(output_, std::move(result));
-                       a->second = std::make_shared<product_store>(
-                         store->index(), this->full_name(), std::move(new_products));
-
-                       message const new_msg{a->second, message_id};
-                       stay_in_graph.try_put(new_msg);
-                       to_output.try_put(new_msg);
-                       flag_for(store->index()->hash()).mark_as_processed();
+                       try {
+                         auto result = call(ft, messages, std::make_index_sequence<N>{});
+                         ++calls_;
+                         ++product_count_[store->index()->layer_hash()];
+                         products new_products;
+                         new_products.add_all(output_, std::move(result));
+                         a->second = std::make_shared<product_store>(
+                           store->index(), this->full_name(), std::move(new_products));
+
+                         message const new_msg{a->second, message_id};
+                         stay_in_graph.try_put(new_msg);
+                         to_output.try_put(new_msg);
+                         flag_for(store->index()->hash()).mark_as_processed();
+                       } catch (...) {
+                         stores_.erase(a);
+                         throw;
+                       }
                      } else {
                        stay_in_graph.try_put({a->second, message_id});
                      }

plugins/python/src/modulewrap.cpp

@@ -405,6 +405,10 @@ namespace {
                                                                                                    \
     auto vec = std::make_shared<std::vector<cpptype>>();                                           \
                                                                                                    \
+    if (!pyobj) {                                                                                  \
+      return vec;                                                                                  \
+    }                                                                                              \
+                                                                                                   \
     /* TODO: because of unresolved ownership issues, copy the full array contents */               \
     if (PyArray_Check((PyObject*)pyobj)) {                                                         \
       PyArrayObject* arr = (PyArrayObject*)pyobj;                                                  \