Don't do math with null pointers in SortingKernel.cpp (#73986)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/73986
Test Plan:
```
buck test -c fbcode.platform=platform010 //dper_lib/silvertorch/modules/filters/tests:external_filter_test -- test_empty_features
```
Reviewed By: malfet
Differential Revision: D34757198
fbshipit-source-id: 35793b40e6e20fea4c8298d4c293ea0b7dbb2b35
(cherry picked from commit 0b5c364e402aff793e47e99d015d657896011965)
