Update check for ctx in constructor

[Giancarlo Anemone]

We need to update the core plugin api to pass null or undefined to the Service constructor when a user calls .of(). Once that is done, we can update our check to check for truthiness rather than ctx.headers.

[Ryan Tsao]

I think this no longer applies with DI @ganemone ?

[old-fusion-bot]

This issue was automatically re-opened because it is referenced in a TODO.