Cleanup: do not use #notNil, use #isNotNil instead

[astares]

We have three cases in Sindarin where #notNil is sent instead of #isNotNil
Our plan is to deprecated #notNil in Pharo 13 so we would like to cleanup all senders first.

see pharo-project/pharo#15118

[astares]

@StevenCostiou would you be able to check and if OK merge this one

[jecisc]

Done