make WriteToArrayOp supporting empty tensor input (#6030)

paddle/operators/tensor_array_read_write_op.cc

@@ -37,9 +37,15 @@ class WriteToArrayOp : public ArrayOp {
                << " to " << offset + 1;
       out->resize(offset + 1);
     }
+    if (x_tensor.memory_size() > 0) {
       auto *out_tensor = &out->at(offset);
       CopyFrom(x_tensor, dev_ctx.GetPlace(), dev_ctx, out_tensor);
       out_tensor->set_lod(x_tensor.lod());
+    } else {
+      VLOG(10) << "WARNING: The input tensor 'x_tensor' holds no memory, so "
+                  "nothing has been written to output array["
+               << offset << "].";
+    }
   }
 };