{transaction,span}.duration.us is unset if {transaction,span}.duration exists

```
{
"script": {
"if": "ctx[ctx.processor.event]?.duration == null",
"source": """def eventDuration = ctx.event?.duration ?: 0;
def rootObjectName = ctx.processor?.event;
def rootObject = ctx[rootObjectName];
if (rootObject == null) {
rootObject = [:];
ctx[rootObjectName] = rootObject;
}
rootObject.duration = ["us": (long)(eventDuration/1000)];
"""
}
}
```
The `if` condition here is too strict that it forbids any other fields under `{transaction,span}.duration`. By default we should never set `{transaction,span}.duration`, but it is still possible via ingest pipelines.