sp_Blitz: Linux quality improvements

[tkwj]

Lending @bmercernccer a helping hand here.
This resolves #3702

Summary:

• Uses the previously established @IsWindowsOperatingSystem variable to determine if certain checks need to be run
• Adds a null check guard against the service account

For reference this is what the related sys.dm_server_services output looks like on SQL Server 2025 Preview on Ubuntu 24.04