Set optional generic type on Pop and MaybePop

[reeichert]

This PR is a fix on Modular Navigator interface, to support optional generic type on Modular.to.pop() and Modular.to.maybePop() to match Navigator's methods.

before:

final String? foo = null;
Modular.to.pop(foo) // Error!, Tried to infer 'Null' for 'T' which doesn't work..

after:

final String? foo = null;
Modular.to.pop(foo) // Works!