Fix [references] no empty refs

clusters/tidal-references.json

@@ -8261,9 +8261,6 @@
       "meta": {
         "date_accessed": "2016-09-13T00:00:00Z",
         "date_published": "2016-09-13T00:00:00Z",
-        "refs": [
-          ""
-        ],
         "source": "MITRE",
         "title": "Bypassing Application Whitelisting using MSBuild.exe - Device Guard Example and Mitigations"
       },
@@ -22573,9 +22570,6 @@
       "meta": {
         "date_accessed": "2018-07-27T00:00:00Z",
         "date_published": "2016-10-20T00:00:00Z",
-        "refs": [
-          ""
-        ],
         "source": "MITRE",
         "title": "How to: Find the Web Application Root"
       },
@@ -43869,9 +43863,6 @@
       "description": "SanDisk. (n.d.). Self-Monitoring, Analysis and Reporting Technology (S.M.A.R.T.). Retrieved October 2, 2018.",
       "meta": {
         "date_accessed": "2018-10-02T00:00:00Z",
-        "refs": [
-          ""
-        ],
         "source": "MITRE",
         "title": "Self-Monitoring, Analysis and Reporting Technology (S.M.A.R.T.)"
       },
@@ -48546,9 +48537,6 @@
       "meta": {
         "date_accessed": "2017-12-20T00:00:00Z",
         "date_published": "2014-07-01T00:00:00Z",
-        "refs": [
-          ""
-        ],
         "source": "MITRE",
         "title": "The Art of Memory Forensics: Detecting Malware and Threats in Windows, Linux, and Mac Memory"
       },

tools/tidal-api/models/cluster.py

@@ -487,7 +487,7 @@ class ReferencesCluster(Cluster):
         for entry in data["data"]:
             meta = ReferencesMeta(
                 source=entry.get("source"),
-                refs=[entry.get("url")],
+                refs=[entry.get("url")] if entry.get("url") != "" else None,
                 title=entry.get("title"),
                 author=entry.get("author"),
                 date_accessed=entry.get("date_accessed"),