[core] Always assign correct uid output in TRefArray::GetObjectUID()

In `TRefArray::GetObjectUID(int &uid, TObjet *obj)`, there is a code
branch for the case of an empty TRefArray and an object with a UID
assigned. Is this case a new process ID corresponding to the object is
assigned to the `TRefArray`. However, the `uid` output parameter is not
correctly assigned in this code branch, which is fixed in this commit.

The consequence of this bug is that the, since the `uid` output
parameter is not correctly assigned, that the implementation of
`TRefArray::Add*()` does not correctly work then.

Closes root-project#12329.

core/cont/src/TRefArray.cxx

@@ -218,6 +218,7 @@ Bool_t TRefArray::GetObjectUID(Int_t &uid, TObject *obj, const char *methodname)
       } else {
          if (GetAbsLast() < 0) {
             // The container is empty, we can switch the ProcessID.
+            uid = obj->GetUniqueID();
             fPID = TProcessID::GetProcessWithUID(obj);
             valid = kTRUE;
             if (gDebug > 3)