unexpected warnings on `std.native` function calls

## Versions

- VS Code Jsonnet Language Server Extension v0.3.3
- jsonnet-language-server v0.11.0

## Example Code

```jsonnet
local nextID = std.native('nextID');

{
  id: nextID(),
}
```

## Error Message

```
RUNTIME ERROR: Unexpected type null, expected function
	/tmp/test.jsonnet:4:7-15	object <anonymous>
	Field "id"	
	During manifestation	
jsonnet evaluation
```