Remove null'ing of context inside cnf.

Author: msporny

contexts/credentials/v2

@@ -44,20 +44,17 @@
     },
     "cnf": {
       "@id": "https://www.iana.org/assignments/jwt#cnf",
-      "@context": [
-        null,
-        {
-          "@protected": true,
-          "kid": {
-            "@id": "https://www.iana.org/assignments/jwt#kid",
-            "@type": "@id"
-          },
-          "jwk": {
-            "@id": "https://www.iana.org/assignments/jwt#jwk",
-            "@type": "@json"
-          }
+      "@context": {
+        "@protected": true,
+        "kid": {
+          "@id": "https://www.iana.org/assignments/jwt#kid",
+          "@type": "@id"
+        },
+        "jwk": {
+          "@id": "https://www.iana.org/assignments/jwt#jwk",
+          "@type": "@json"
         }
-      ]
+      }
     },
     "_sd_alg": {
       "@id": "https://www.iana.org/assignments/jwt#_sd_alg"