fix(ci-sizer): 🐛 use safe map access for optional GARM_URL env var

`index .Env "GARM_URL"` returns empty string for missing keys instead
of panicking with "map has no entry for key".

Ref: IPCEICIS-6886

template/stacks/ci-sizer/sizer-receiver/deployment.yaml

@@ -39,7 +39,7 @@ spec:
                 secretKeyRef:
                   name: sizer-tokens
                   key: hmac-key
-{{{ if .Env.GARM_URL }}}
+{{{ if index .Env "GARM_URL" }}}
             - name: GARM_URL
               value: "{{{ .Env.GARM_URL }}}"
             - name: GARM_USER