[v2.12.0] `recipe_zmnam.yml` diagnostic failure

[sloosvel]

During the recipe testing for version 2.12.0 (#3916), recipe_zmnam.yml failed to run the diagnostics with the following error:

Traceback (most recent call last):
  File "/home/b/b381943/release/ESMValTool/esmvaltool/diag_scripts/zmnam/zmnam.py", line 129, in <module>
    main(config)
  File "/home/b/b381943/release/ESMValTool/esmvaltool/diag_scripts/zmnam/zmnam.py", line 98, in main
    plot_files = zmnam_plot(file_mo_an, out_dir + '/',
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/b/b381943/release/ESMValTool/esmvaltool/diag_scripts/zmnam/zmnam_plot.py", line 215, in zmnam_plot
    for cmap in inv_map.collections:
                ^^^^^^^^^^^^^^^^^^^
AttributeError: 'GeoContourSet' object has no attribute 'collections'

• See the logs at: https://esmvaltool.dkrz.de/shared/esmvaltool/v2.12.0/recipe_zmnam_20250224_082909/run/