add nil check in if statement configmap example, also note why

docs/chart_template_guide/control_structures.md

@@ -53,10 +53,10 @@ data:
   myvalue: "Hello World"
   drink: {{ .Values.favorite.drink | default "tea" | quote }}
   food: {{ .Values.favorite.food | upper | quote }}
-  {{ if eq .Values.favorite.drink "coffee" }}mug: true{{ end }}
+  {{ if (.Values.favorite.drink) and eq .Values.favorite.drink "coffee" }}mug: true{{ end }}
 ```
 
-Since we commented out `drink: coffee` in our last example, the output should not include a `mug: true` flag. But if we add that line back into our `values.yaml` file, the output should look like this:
+Note that `.Values.favorite.drink` must be defined or else it will throw an error when comparing it to "coffee". Since we commented out `drink: coffee` in our last example, the output should not include a `mug: true` flag. But if we add that line back into our `values.yaml` file, the output should look like this:
 
 ```yaml
 # Source: mychart/templates/configmap.yaml