allow null in character and corporation partials

[recursivetree]

Allows you to enter null into the character and corporation partials. My use case is writing something like this:

@include("web::partials.character",["character"=>$order->user->main_character ?? null])

This pr is intended to be deployed together with eveseat/services#157, but it will work even if the other pr is not merged, because we have null to int conversion in the img() function.