`run_rosetta()` fails to create batch sequence

[brownag]

Reported by @ColbyBrungard

library(terra)
terraOptions(memfrac=0.9, progress=2)

# test SpatRast sand, silt, clay – all sum to exactly 100%
test = rast("Z:\\vf\\WaterRetentionDifference\\test/sum_30cm_test_fix.tif")

run_rosetta(soildata = test,
  vars = NULL,
  rosetta_version = 2,
  cores = 8,
  file = "awc_30_cm.tif",
  nrows = 10000, 
  overwrite = TRUE
)

Yields the following:

Error in seq.default(1, out_info$nrows, nrows) : 'to' must be of length 1

TODO:

• [ ] fall back for empty out_info object from writeStart() (why does this happen? because of #11/inability to open file for writing? other reasons?)
• [x] better nrows heuristic (borrow nrow(.data)/(terra::ncell(.data)/core_thresh) from jNSMR)
• [ ] nrows: consider update of magic number for rosetta v.s. jNSM (core_thresh=25000L)