hotspot_gistar() reports automatically selected cell size even when grid is provided

[mpjashby]

Describe the bug
hotspot_gistar() reports automatically selected cell size even when grid is provided

To Reproduce

library(sfhotspot)

memphis_grid <- memphis_population |> 
  sf::st_transform("EPSG:2843") |> 
  hotspot_grid(cell_size = 500)

memphis_gistar <- memphis_robberies_jan |> 
  sf::st_transform("EPSG:2843") |> 
  hotspot_gistar(kde = FALSE, grid = memphis_grid)
#> Cell size set to 500 metres automatically

^{Created on 2024-03-20 with reprex v2.1.0}

Expected behavior
A message reporting the automatically selected value of cell_size should only be produced when grid and cell_size are NULL.

[mpjashby]

This is specific to hotspot_gistar() because in other other functions create_grid() is only called if the grid argument is NULL, but for hotspot_gistar() the grid is calculated first before being used to calculate both KDE and GI* values.