export_dreq_lists_json fails for opportunities with no core variables

[bthrasher]

Hello again! This is somewhat related to my previous issue (#196). When using the export_dreq_lists_json command line function, the dq.get_requested_variables call is forced to check for core variables (check_core_variables=True), so it fails for any opportunities that do not include those (e.g., 20 - Core Climate Services):

export_dreq_lists_json -i 20 v1.2.2.2 test.json

Traceback (most recent call last):
File "/Library/Frameworks/Python.framework/Versions/3.14/bin/export_dreq_lists_json", line 7, in
sys.exit(main())
~~~~^^
File "/Library/Frameworks/Python.framework/Versions/3.14/lib/python3.14/site-packages/data_request_api/command_line/export_dreq_lists_json.py", line 149, in main
expt_vars = dq.get_requested_variables(base, use_dreq_version,
use_opps=use_opps, priority_cutoff=args.priority_cutoff,
verbose=False)
File "/Library/Frameworks/Python.framework/Versions/3.14/lib/python3.14/site-packages/data_request_api/query/dreq_query.py", line 616, in get_requested_variables
raise ValueError(msg)
ValueError: Empty Core variables list for experiment: scenariomip14

[sol1105]

Hi Bridget, the check exists to prevent users from unintentionally omitting the core/BSV variables. I agree it would be useful to bypass this check occasionally to look at the isolated requests from certain opportunities.
In #170 I suggest a command line option for that purpose:

    parser.add_argument("-d", "--disable_core_vars_check", action="store_true", default=False,
                        help="Don't require that core variables are included in the request.")

[YanchunHe]

When checking out specific experiment, the 'forced checking' will throw errow on other experiments without core variables, which does not make much sense:

$ export_dreq_lists_json -p core -i 55 -m metadata.json -e historical v1.2.2 test.json

Traceback (most recent call last):
  File "/cmip7_dreq_env/bin/export_dreq_lists_json", line 7, in <module>
    sys.exit(main())
             ~~~~^^
  File "/cmip7_dreq_env/lib/python3.13/site-packages/data_request_api/command_line/export_dreq_lists_json.py", line 149, in main
    expt_vars = dq.get_requested_variables(base, use_dreq_version,
                                           use_opps=use_opps, priority_cutoff=args.priority_cutoff,
                                           verbose=False)
  File "//cmip7_dreq_env/lib/python3.13/site-packages/data_request_api/query/dreq_query.py", line 634, in get_requested_variables
    raise ValueError(msg)
ValueError: Empty Core variables list for experiment: 1pctCO2-bgc