class_docstr: Test basic functionality of AWIPS Tiled writer.
class_docstr_tok: ["Test","basic","functionality","of","AWIPS","Tiled","writer","."]
docstr: Test creating a lettered grid with no valid data.
docstr_tok: ["Test","creating","a","lettered","grid","with","no","valid","data","."]
code: def test_lettered_tiles_no_valid_data(self, tmp_path):
      """Test creating a lettered grid with no valid data."""
      from satpy.writers.awips_tiled import AWIPSTiledWriter
  
      w = AWIPSTiledWriter(base_dir=str(tmp_path), compress=True)
      data = da.full((2000, 1000), np.nan, chunks=500, dtype=np.float32)
      area_def = _get_test_area(
          shape=(2000, 1000),
          extents=(-1000000.0, -1500000.0, 1000000.0, 1500000.0),
      )
      ds = _get_test_lcc_data(data, area_def)
      w.save_datasets(
          [ds],
          sector_id="LCC",
          source_name="TESTS",
          tile_count=(3, 3),
          lettered_grid=True,
      )
      # No files created - all NaNs should result in no tiles being created
      all_files = glob(os.path.join(str(tmp_path), "TESTS_AII*.nc"))
      assert not all_files
  
imports: import logging
  import os
  import shutil
  from datetime import datetime, timedelta
  from glob import glob
  import dask
  import dask.array
  import numpy
  import pytest
  import xarray
  from pyproj import CRS
  from satpy.resample import update_resampled_coords
label_desc: Use image node_id 10 for calling the TestAWIPSTiledWriter obj's underlying member method code with example usage: obj.test_lettered_tiles_no_valid_data(tmp_path) without return types