extract_json_strings.py borked on optional bodyparts name

[scarf005]

Describe the bug

workflow

Run lang/update_pot.sh
> Extracting strings from json
==> Generating the list of all Git tracked files
==> Parsing JSON
----> Traversing directory data/help
----> Traversing directory data/json
Traceback (most recent call last):
  File "/home/runner/work/Cataclysm-BN/Cataclysm-BN/lang/extract_json_strings.py", line 1268, in <module>
    extract_all_from_dir(state, i)
  File "/home/runner/work/Cataclysm-BN/Cataclysm-BN/lang/extract_json_strings.py", line 1178, in extract_all_from_dir
    extract_all_from_file(state, full_name)
  File "/home/runner/work/Cataclysm-BN/Cataclysm-BN/lang/extract_json_strings.py", line 1200, in extract_all_from_file
    extract(state, jsonobject)
  File "/home/runner/work/Cataclysm-BN/Cataclysm-BN/lang/extract_json_strings.py", line 1001, in extract
    extract_specials[object_type](state, item)
  File "/home/runner/work/Cataclysm-BN/Cataclysm-BN/lang/extract_json_strings.py", line 231, in extract_bodypart
    writestr(state, item["heading_multiple"])
KeyError: 'heading_multiple'
Error in extract_json_strings.py. Aborting
Error: Process completed with exit code 1.

Cataclysm-BN/lang/extract_json_strings.py

Lines 229 to 231 in 701016c

	writestr(state, item["encumbrance_text"])
	writestr(state, item["heading"])
	writestr(state, item["heading_multiple"])

#2775 made bodypart names like heading_multiple optional, however extract_json_strings.py doesn't know that and it's been secretly failing for a month.

Steps To Reproduce

run lang/update_pot.sh from project root

Additional context

originally reported in discord by fruitybite

[scarf005]

cc @KheirFerrum