fix(helm): gate the DD_* env-vars test on those values being set

templates/tests/test-env-vars.yaml is a helm.sh/hook: test pod that
asserts DD_ENV == dev_helm, DD_SERVICE == litellm and
USE_DDTRACE == true, reading them from .Values.envVars. Those values
are only set in ci/test-values.yaml, which the chart's own CircleCI
job uses. values.yaml defaults envVars to {}, so on every real
install that doesn't set the Datadog env vars, `helm test <release>`
fails with "Environment variable DD_ENV mismatch. Expected: dev_helm,
Got: ", and since helm stops at the first failed test, it also masks
the useful test-connection test.

Render the pod only when all three values are actually set. CI
behavior is unchanged since ci/test-values.yaml sets them.

helm/litellm-helm/templates/tests/test-env-vars.yaml

@@ -1,3 +1,5 @@
+{{- $envVars := .Values.envVars | default dict }}
+{{- if and $envVars.DD_ENV $envVars.DD_SERVICE $envVars.USE_DDTRACE }}
 apiVersion: v1
 kind: Pod
 metadata:
@@ -40,4 +42,5 @@ spec:
           value: {{ .Values.envVars.DD_SERVICE | quote }}
         - name: USE_DDTRACE
           value: {{ .Values.envVars.USE_DDTRACE | quote }}
-  restartPolicy: Never
+  restartPolicy: Never
+{{- end -}}