Add assertions to validate parsed schema cache keys

Author: rayokota

core/src/main/java/io/confluent/kafka/schemaregistry/storage/KafkaSchemaRegistry.java

@@ -2893,6 +2893,8 @@ private static class RawSchema {
     private final boolean normalize;
 
     public RawSchema(Schema schema, boolean isNew, boolean normalize) {
+      assert schema != null && schema.getVersion() == null && schema.getId() == null;
+
       this.schema = schema;
       this.isNew = isNew;
       this.normalize = normalize;