Latest to date chart release break upgrade

[gemelen]

The changeset introduced via #168 assumes that podMonitor exists and its enabled property is set.
That isn't always true and if the assumption is broken, the upgrade fails.

For my case in particular, upgrade from 7.6.0 to 7.6.1 failed, but the most of the values set is rolling from at least the version 7.0.0 (I don't remember the original 6.x version with which I've started).

Error is observed as

helm upgrade --install rocketchat rocketchat/rocketchat \                        
  --namespace rocketchat \                                                                                                                                                             
  --description "Upgrade to 7.6.1" \                                                                                                                                                   
  --reuse-values \                                                                                                                                                                     
  --values ./values.yaml \                                                                                                                                                             
  --atomic --cleanup-on-fail --debug                                                                                                                                                   
                                                                                                                                                                                      
history.go:56: 2025-05-23 22:33:17.895971961 -0300 -03 m=+0.858494816 [debug] getting history for release rocketchat                                                                   
upgrade.go:164: 2025-05-23 22:33:19.534089532 -0300 -03 m=+2.496612291 [debug] preparing upgrade for rocketchat                                                                        
upgrade.go:561: 2025-05-23 22:33:20.005235437 -0300 -03 m=+2.967758186 [debug] reusing the old release's values                                                                        
Error: UPGRADE FAILED: template: rocketchat/templates/podmonitor.yaml:24:18: executing "rocketchat/templates/podmonitor.yaml" at <.Values.podMonitor.enabled>: nil pointer evaluating i
nterface {}.enabled                                                                                                                                                                    
helm.go:86: 2025-05-23 22:33:21.018835059 -0300 -03 m=+3.981357806 [debug] template: rocketchat/templates/podmonitor.yaml:24:18: executing "rocketchat/templates/podmonitor.yaml" at <.
Values.podMonitor.enabled>: nil pointer evaluating interface {}.enabled                                                                                                                
UPGRADE FAILED                                                                                                                                                                         
main.newUpgradeCmd.func2                                                                                                                                                               
        helm.sh/helm/v3/cmd/helm/upgrade.go:244                                                                                                                                        
github.com/spf13/cobra.(*Command).execute
        github.com/spf13/cobra@v1.8.1/command.go:985
github.com/spf13/cobra.(*Command).ExecuteC
        github.com/spf13/cobra@v1.8.1/command.go:1117
github.com/spf13/cobra.(*Command).Execute
        github.com/spf13/cobra@v1.8.1/command.go:1041
main.main
        helm.sh/helm/v3/cmd/helm/helm.go:85
runtime.main
        runtime/proc.go:272
runtime.goexit
        runtime/asm_amd64.s:1700

with values.yaml as

image:
  repository: registry.rocket.chat/rocketchat/rocket.chat
  tag: 7.6.1