tm_grid fails

It seems to be related to some of the recent changes -- it worked well a few days ago:

``` r
library(tmap)
library(spData)

tm_shape(nz) +
  tm_polygons() +
  tm_grid()
#> Error in vapply(co.x, nrow, FUN.VALUE = integer(1)): values must be length 1,
#>  but FUN(X[[1]]) result is length 0
```

<sup>Created on 2025-07-07 with [reprex v2.1.1](https://reprex.tidyverse.org)</sup>