Broken citations can bypass our citation tests

Because of how citations work, in some places broken citations will show up as an empty reference in the external references table without leaving (Citation: on the built page. This means our citation tests never detects an issue.

To reproduce, replace APT18 in the STIX data with this intentionally broken object:

        {
            "created_by_ref": "identity--c78cb6e5-0c4b-4611-8297-d1b8b55e40b5",
            "name": "APT18",
            "description": "[APT18](https://attack.mitre.org/groups/G0026) is a threat group that has operated since at least 2009 and has targeted a range of industries, including technology, manufacturing, human rights groups, government, and medical. (Citation: Dell Lateral Movemente)",
            "type": "intrusion-set",
            "aliases": [
                "APT18",
                "TG-0416",
                "Dynamite Panda",
                "Threat Group-0416"
            ],
            "object_marking_refs": [
                "marking-definition--fa42a846-8d90-4e51-bc29-71d5b4802168"
            ],
            "id": "intrusion-set--38fd6a28-3353-4f2b-bb2b-459fecd5c648",
            "external_references": [
                {
                    "external_id": "G0026",
                    "source_name": "mitre-attack",
                    "url": "https://attack.mitre.org/groups/G0026"
                },
                {
                    "source_name": "APT18",
                    "description": "(Citation: ThreatStream Evasion Analysis)(Citation: Anomali Evasive Maneuvers July 2015)"
                },
                {
                    "source_name": "TG-0416",
                    "description": "(Citation: ThreatStream Evasion Analysis)(Citation: Anomali Evasive Maneuvers July 2015)"
                },
                {
                    "source_name": "Dynamite Panda",
                    "description": "(Citation: ThreatStream Evasion Analysis)(Citation: Anomali Evasive Maneuvers July 2015)"
                },
                {
                    "source_name": "Threat Group-0416",
                    "description": "(Citation: ThreatStream Evasion Analysis)"
                },
                {
                    "source_name": "Dell Lateral Movement",
                    "description": "Carvey, H.. (2014, September 2). Where you AT?: Indicators of lateral movement using at.exe on Windows 7 systems. Retrieved January 25, 2016.",
                    "url": "http://www.secureworks.com/resources/blog/where-you-at-indicators-of-lateral-movement-using-at-exe-on-windows-7-systems/"
                },
                {
                    "source_name": "ThreatStream Evasion Analysis",
                    "description": "Shelmire, A.. (2015, July 6). Evasive Maneuvers. Retrieved January 22, 2016.",
                    "url": "https://www.threatstream.com/blog/evasive-maneuvers-the-wekby-group-attempts-to-evade-analysis-via-custom-rop"
                },
                {
                    "source_name": "Anomali Evasive Maneuvers July 2015",
                    "description": "Shelmire, A. (2015, July 06). Evasive Maneuvers by the Wekby group with custom ROP-packing and DNS covert channels. Retrieved November 15, 2018.",
                    "url": "https://www.anomali.com/blog/evasive-maneuvers-the-wekby-group-attempts-to-evade-analysis-via-custom-rop"
                }
            ],
            "modified": "2019-05-30T18:05:32.461Z",
            "x_mitre_version": "2.0",
            "created": "2017-05-31T21:31:57.733Z"
        },

Example of how this is represented:

mitre-attack / attack-website

Broken citations can bypass our citation tests #21