* GSNSignatureBoucer does not accept zero address

* Linting code.

* Update contracts/GSN/bouncers/GSNBouncerSignature.sol

Makes sense!

Co-Authored-By: Nicolás Venturo <nicolas.venturo@gmail.com>

* Update test/GSN/GSNBouncerSignature.test.js

ok!

Co-Authored-By: Nicolás Venturo <nicolas.venturo@gmail.com>

* Add zero address constant from OZ test Helpers

* revert prettier formatting