Ensure that a @context array item is a string or a non-empty object.

Co-authored-by: Manu Sporny <msporny@digitalbazaar.com>

Author: absoludity

schema/verifiable-credential/verifiable-credential-schema.json

@@ -161,12 +161,31 @@
   "properties": {
     "@context": {
       "type": "array",
+      "contains": {
+        "const": "https://www.w3.org/ns/credentials/v2"
+      },
+      "minItems": 1,
+      "items": {
+        "oneOf": [
+          {
+            "type": "string",
+            "minLength": 1
+          },
+          {
+            "type": "object",
+            "minProperties": 1,
+            "propertyNames": {
+              "type": "string",
+              "minLength": 1
+            }
+          }
+        ]
+      },
       "prefixItems": [
         {
           "const": "https://www.w3.org/ns/credentials/v2"
         }
       ],
-      "minItems": 1,
       "uniqueItems": true
     },
     "id": {