fpr_sp_assign_utm fails with empty rows, need to build safety checks

[lucy-schick]

fpr_sp_assign_utm fails when dat contains empty rows because it cannot find a crs due to NAs.

Need to build in a check to make sure dat doesn't contain any empty rows, and inform user about whats going on.

Found this problem when calling fpr_sp_gpkg_backup which calls fpr_sp_assign_utm:

> # first we back up the gpkg in the repo and update the coordinate columns in the gpkg in QGIS
> pscis_export_raw <- fpr_sp_gpkg_backup(
+   path_gpkg = "~/Projects/gis/sern_lchl_necr_fran_2023/data_field/2023/form_pscis_2023.gpkg",
+   update_utm = TRUE,
+   update_site_id = TRUE,
+   write_back_to_path = TRUE,
+   write_to_csv = TRUE,
+   # this versions on git everytime due to metadata and can't be tracked visually. Should only be committed when
+   # csv is versioned
+   write_to_rdata = TRUE,
+   return_object = TRUE)

Error in `purrr::map2()`:
ℹ In index: 3.
Caused by error:
! crs not found: is it missing?

Removed the 3 empty rows from form_pscis_2023.gpkg in Q and then it worked.