fix(helm): Fix helm rendering error when upgrading

[adleong]

Fixes #14228

When upgrading from a previous version of Linkerd, the proxy.tracing key will not exist in the Linkerd config ConfigMap. If a user attempts to use the inject CLI command in --manual mode, the config will be loaded from the Linkerd config ConfigMap and then rendering of the pod patch template will fail:

template: patch/templates/patch.json:106:16: executing "patch/templates/patch.json" at <.Values.proxy.tracing.enable>: nil pointer evaluating interface {}.enable

This is because the template is attempting to access the enabled key in the tracing struct which is nil.

We add guards using the dig Helm template function so that template rendering does not fail, even if the proxy.tracing key is not present.