oc_check_query for NAs in placename or latitude/longitude

[dpprdan]

As of recently, OpenCage throws a HTTP 400 ‘bad query’ error when the query is empty, so we cannot be as sneaky as in #89 anymore.
I.e. now we need to make sure that we do not send empty queries (empty placename
or latitude/longitude) to the API, but alert the user beforehand. This is what happens with this PR.

An empty request throws an HTTP 400 error (shown with oc_process() here, but the same happens with oc_forward() and oc_reverse() without this fix):

library(opencage)
opencage:::oc_process("", return = "df_list")
#> Error: HTTP failure: 401
#> missing API key

Instead, oc_check_query() now throws an error before moving to oc_process()/calling the API:

oc_forward("")
#> Error: `placename` must not be NA or an empty string.
oc_reverse(NA_real_, NA_real_)
#> Error: `latitude` must not be NA.

I was a bit reluctant to let the whole call error out ‘just’ because a user provides an empty/NA placename/coordinates, but this is a) what the API effectively demands now and b) essentially the same as providing incorrect coordinates, e.g. outside the (-)90/(-)180 bounds:

oc_reverse(100, 200)
#> Error: Every `latitude` must be between -90 and 90.

[maelle]

Thanks a lot! 🚀