component: Mark relevant return types as nullable
component: Mark relevant return types as nullable

These types are not marked as nullable in the resulting GIR file and hence not marked nullable in any bindings downstream of that; for example, the Vala bindings.

Having these marked as nullable would allow more robust Vala code to be generated when these methods are used.

This only touches Component for now in case there's some reasoning behind why these haven't been marked `nullable`, but I'm happy to submit a PR to cover off the rest if this is wanted.
ximion/appstream,ximion/appstream,ximion/appstream,ximion/appstream