tor: test_keys are empty in some failure conditions

I found a measurement with an empty test_keys field:

{'annotations': {'engine_name': 'ooniprobe-engine',
                 'engine_version': '3.10.0-beta.3',
                 'platform': 'linux'},
 'data_format_version': '0.2.0',
 'input': None,
 'measurement_start_time': '2022-01-01 02:12:21',
 'probe_asn': 'AS57269',
 'probe_cc': 'ES',
 'probe_ip': '127.0.0.1',
 'probe_network_name': 'DIGI SPAIN TELECOM S.L.U.',
 'report_id': '20220101T021219Z_tor_ES_57269_n1_JFoQMFGqYirwPXOo',
 'resolver_asn': 'AS57269',
 'resolver_ip': '2a0c:5a84:0:1::2',
 'resolver_network_name': 'DIGI SPAIN TELECOM S.L.U.',
 'software_name': 'iThena-ooniprobe',
 'software_version': '1.0.0',
 'test_keys': None,
 'test_name': 'tor',
 'test_runtime': 15.052319403,
 'test_start_time': '2022-01-01 02:12:19',
 'test_version': '0.3.0'}

Ideally we would never generate a result of this sort, so I think this deserves some investigation.