code: def custom_labeller(cell):
      if cell.data is np.nan:
          return "missing"
  
file_path: statsmodels/statsmodels/iolib/tests/test_table_econpy.py
imports: import numpy
  from numpy.testing import assert_equal
  from statsmodels.iolib.table import Cell, SimpleTable
  from statsmodels.iolib.table import default_latex_fmt
  from statsmodels.iolib.table import default_html_fmt
label_desc: Use image node_id 1 for calling a global function with example usage: custom_labeller(cell) and returns: str