Fix nil pointer evaluating interface

[fl0-m]

Upgrading to 0.23.0, I get this error

Error: template: trino/templates/ingress.yaml:33:25: executing "trino/templates/ingress.yaml" at <include "trino.fullname" .>: error calling include: template: trino/templates/_helpers.tpl:15:14: executing "trino.fullname" at <.Values.fullnameOverride>: nil pointer evaluating interface {}.fullnameOverride
helm.go:84: [debug] template: trino/templates/ingress.yaml:33:25: executing "trino/templates/ingress.yaml" at <include "trino.fullname" .>: error calling include: template: trino/templates/_helpers.tpl:15:14: executing "trino.fullname" at <.Values.fullnameOverride>: nil pointer evaluating interface {}.fullnameOverride

This PR aims to fix nil pointer evaluating interface, by using $, which references the global scope in order to access the Values as expected.

[fl0-m]

@nineinchnick 👋 PTAL

[nineinchnick]

The same fix was suggested in #179. I'm inclined to merge this first since you have the CLA signed, and I could release a new version quicker.

[nineinchnick]

Can you also add a test, so we can avoid regressions like this in future?

[fl0-m]

Sure

[fl0-m]

@nineinchnick PTAL