Initialize PortableHostObject::product_

OutputModules can construct the PortableHostObject via the
"uninitialized" constructor if no producer produced the data product.
Explicit null value for the pointer communicates the absence of data
to ROOT.

Author: makortel

DataFormats/Portable/interface/PortableHostObject.h

@@ -112,7 +112,7 @@ class PortableHostObject {
 
 private:
   std::optional<Buffer> buffer_;  //!
-  Product* product_;
+  Product* product_ = nullptr;
 };
 
 #endif  // DataFormats_Portable_interface_PortableHostObject_h

DataFormats/Portable/interface/PortableHostObjectReadRules.h

@@ -33,8 +33,13 @@ static void readPortableHostObject_v1(char *target, TVirtualObject *from_buffer)
   // pointer to the Object object being constructed in memory
   Object *newObj = (Object *)target;
 
-  // move the data from the on-file layout to the newly constructed object
-  Object::ROOTReadStreamer(newObj, *onfile.product_);
+  // product_ can be null if the Wrapper<PortableHostObject<T>> was
+  // default-constructed because a producer did not produce the
+  // product
+  if (onfile.product_) {
+    // move the data from the on-file layout to the newly constructed object
+    Object::ROOTReadStreamer(newObj, *onfile.product_);
+  }
 }
 
 // put set_PortableHostObject_read_rules in the ROOT namespace to let it forward declare GenerateInitInstance