Throw error for NULL labels passed to callouts

`gsplot() %>% callouts(NA, NA, labels=NULL)` produces the error:
```
Error in text.default(x = numeric(0), y = numeric(0), labels = NULL, pos = c(1,  : 
  no coordinates were supplied
```
I believe it's caused by labels being null, but a more explicit error message helping the caller identify the issue would be useful.