Helm: Tail url looks incorrect

[slim-bean]

          not sure if this is intended but in the [_helpers.tpl](https://github.com/grafana/loki/blob/main/production/helm/loki/templates/_helpers.tpl#L955C8-L955C39) there is an if check which might be wrong: 

{{- if "loki.deployment.isDistributed "}}

similar check is done here which looks like this:

{{- $isDistributed := eq (include "loki.deployment.isDistributed" .) "true" -}}
{{- if $isDistributed -}}

This causes the if check to always be true and thus the frontend.tail_proxy_url to be set in the loki config. But the configured tail_proxy_url does not point to an existing service (I used SSD deplyoment mode). Not sure if this has any impact.

Originally posted by @dakr0013 in #12506 (comment)