spot-termination-exporter: Initialize nameOverride option

This fixes the following problem with 'helm lint --strict':

==> Linting spot-termination-exporter/
[INFO] Chart.yaml: icon is recommended
[ERROR] templates/: render error in "spot-termination-exporter/templates/service.yaml": template: spot-termination-exporter/templates/_helpers.tpl:14:40: executing "spotTerminationexporter.fullname" at <.Values.nameOverride>: map has no entry for key "nameOverride"

Author: hwoarang

spot-termination-exporter/values.yaml

@@ -54,6 +54,8 @@ resources:
     memory: 256Mi
     cpu: 120m
 
+nameOverride: ""
+
 nodeSelector: {}
 
 tolerations: []