I have this

```
let envIfNotNull = n: k: v: optionalAttrs (n != null) { k = v; };
in envIfNotNull subcfg.tracking-id "INTRAY_WEB_SERVER_ANALYTICS_TRACKING_ID" ""${subcfg.tracking-id}"
```

and that somehow evaluates to an empty attribute set even when subcfg.tracking-id is not null
while this doesn't:

```
optionalAttrs (subcfg.tracking-id != null) {
                  "INTRAY_WEB_SERVER_ANALYTICS_TRACKING_ID" = "${subcfg.tracking-id}";}
```