ingress.yaml template returns error in renderring –> nil pointer evaluating interface {}.service

What you’re seeing is a weird caveat in Go templating. Your conditional logic is being evaluated inside a range loop. This means . you’re using to access Values is not the one you expect it to be, as it’s overridden for each range iteration evaluation.
You can use $, which references the global scope in order to access the Values as expected.

For your scenario, it would be something like:

- path: {{ default "/" .path | quote }}
        backend:
        {{- if $.Values.service.servicename }}
          serviceName: {{ $.Values.service.servicename }}
        {{- else }}
          serviceName: {{ include "chartmuseum.fullname" $ }}
        {{- end }}

See here for more details.