[Mellanox] Remove external-changes patch if no downstream changes are found

[vivekrnv]

Why I did it

Bug Fix: when the external-changes.patch is empty, sonic-linux-kernel build fails when called with INCLUDE_EXTERNAL_PATCHES=y flag

Work item tracking

• Microsoft ADO (number only):

How I did it

How to verify it

Verified by integrating a hw-mgmt version without downstream patches and building linux kernel with EXTERNAL_PATCHES flag

Which release branch to backport (provide reason below if selected)

• [ ] 201811
• [ ] 201911
• [ ] 202006
• [ ] 202012
• [ ] 202106
• [ ] 202111
• [ ] 202205
• [ ] 202211
• [ ] 202305

Tested branch (Please provide the tested image version)

• [ ]
• [ ]

Description for the changelog

Link to config_db schema for YANG module changes

A picture of a cute animal (not mandatory but encouraged)

[xumia]

/azp run Azure.sonic-buildimage

[azure-pipelines[bot]]

Azure Pipelines successfully started running 1 pipeline(s).

[vivekrnv]

/azpw run Azure.sonic-buildimage

[mssonicbld]

/AzurePipelines run Azure.sonic-buildimage

[azure-pipelines[bot]]

Azure Pipelines successfully started running 1 pipeline(s).

[linux-foundation-easycla[bot]]

The committers listed above are authorized under a signed CLA.

• :white_check_mark: login: vivekrnv / name: Vivek (570171d9e5ac1ec7a54ab520ff2d55bc14bb9894, de98b411718d4fa5ea05ca38ce8341258f63c670, 9dc3d512df4b94229c9c708360e1a870f9cc7d67)

[mssonicbld]

Cherry-pick PR to 202311: https://github.com/sonic-net/sonic-buildimage/pull/19797