Replace '== null' with 'is null' and '!= null' with 'is not null'
across 19 core files following modern C# pattern matching conventions.
Only literal null comparisons are changed. Assignments, value
comparisons, and LINQ expressions are left untouched.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Fetching updates return an error because the UpgradeHelper returns an invalid RID, and we use the RID to create automated builds. This updates the UpgradeHelper to be compliant with the catalog.
