Fix push script passing in optional argument (#27591)
Fix push script passing in optional argument (#27591)

With new PS command line parsing update in 7.3 they explicitly
pass "" to the native commands which in our usage here we don't
want. However setting the variable to null instead works for our
scenario with both command line parsing strategies so using it here.

Co-authored-by: Wes Haggard <d598ed404a9a39e5ca39f22624afbc827cbec5f5@microsoft.com>
Azure/azure-sdk-for-python,Azure/azure-sdk-for-python,Azure/azure-sdk-for-python,Azure/azure-sdk-for-python