chore: improve error when specifying no fields

repro from test that used to work: 

```r
  furl <- paste0(
    "https://services.arcgis.com/P3ePLMYs2RVChkJx/arcgis/rest/services/",
    "USA_Wetlands/FeatureServer/1"
  )

  flayer <- arc_open(furl)
expect_no_error(arc_select(flayer, fields = "", n_max = 100))
```