C#: Check if JetBrains Rider editor path is empty

[raulsntos]

• Add check to see if the path to Rider is empty before attempting to start a process. If so, show a better error to the user.
  • Addresses The Rider option does not start Rider, but Custom does. #78501.
• Enable nullability in RiderPathManager and remove unnecessary null checks when we know the string can't be null.
• Make EditorPathSettingName a private constant, it doesn't seem like it was being used anywhere outside of this class.

I don't have Rider so I couldn't test but the changes seem safe.

[YuriSizov]

Looks like a good code improvement.

[akien-mga]

Thanks!