Validate pipeline fails for PR with custom sorting

[Agger1995]

Describe the bug
In an AzOps repository, with the configuration AZOPS_CUSTOM_SORT_ORDER=true, i.e. custom sorting is enabled.
The Validate pipeline fails for pull-requests where the change set only consists of modifications to a .order file.

This happens because the CustomSorting step of the pipeline modifies the /tmp/diff.txt file to be empty, when there is only a change to the .order file.

Relevant script: https://github.com/Azure/AzOps-Accelerator/blob/main/.scripts/customSorting.ps1#L26C1-L28C6

Steps to reproduce

1. Create an AzOps Accelerator repository, to track Azure resources at any level
2. Enable custom sorting, by setting environment variable AZOPS_CUSTOM_SORT_ORDER=true
3. Create a PR, modifying only a .order file, at any tracked level in the AzOps repository
4. Execute the Validate pipeline for the PR

Pipeline outputs:
CustomSorting pipeline step:

Files found in diff:
M	root/my management group (00000000-0000-0000-0000-000000000000)/my subscription (00000000-0000-0000-0000-000000000000)/.order
Sorted files:

Validate pipeline step error:

Cannot bind argument to parameter 'ChangeSet' because it is null.
At /__w/_temp/52355258-7ba8-4086-8efb-8bdca5da3788.ps1:15 char:31
+   Invoke-AzOpsPush -ChangeSet $diff -WhatIf:$RunWhatIf -CustomSortOrd …
+                               ~~~~~

##[error]PowerShell exited with code '1'.