fix nuls values in not required but with format

schemas/course-credential.json

@@ -117,7 +117,7 @@
                             "description": "The number of (ECTS) credits awarded for the course"
                         },
                         "dateOfAssessment": {
-                            "type": "string",
+                            "type": ["string", "null"],
                             "description": "The date of assessment",
                             "format": "date"
                         },

schemas/federation-membership.json

@@ -100,7 +100,7 @@
               "type": "string"
             },
             "website": {
-              "type": "string",
+              "type": ["string", "null"],
               "format": "uri"
             },
             "evidence": {

schemas/membership-card.json

@@ -52,7 +52,7 @@
               "minLength":  1
             },
             "affiliatedUntil": {
-              "type": "string",
+              "type": ["string", "null"],
               "format": "date"
             },
             "typeOfPerson": {