[stable/prometheus] Fix node exporter args (#3585)

* Fix node_exporter args to access keys without values to support the new collector enabling logic

* Update Chart.yaml

stable/prometheus/Chart.yaml

@@ -1,5 +1,5 @@
 name: prometheus
-version: 5.1.1
+version: 5.1.2
 description: Prometheus is a monitoring system and time series database.
 home: https://prometheus.io/
 icon: https://raw.githubusercontent.com/prometheus/prometheus.github.io/master/assets/prometheus_logo-cb55bb5c346.png

stable/prometheus/templates/node-exporter-daemonset.yaml

@@ -34,7 +34,11 @@ spec:
             - --path.procfs=/host/proc
             - --path.sysfs=/host/sys
           {{- range $key, $value := .Values.nodeExporter.extraArgs }}
+          {{- if $value }}
             - --{{ $key }}={{ $value }}
+          {{- else }}
+            - --{{ $key }}
+          {{- end }}
           {{- end }}
           ports:
             - name: metrics