Deserializing Json null value makes TLC (silently) throw

NullPointerExceptions, potentially causing massive slow down.

Fixes Github issue #868
tlaplus/tlaplus#868

[Bug]

Signed-off-by: Markus Alexander Kuppe <github.com@lemmster.de>

modules/tlc2/overrides/Json.java

@@ -406,9 +406,6 @@ else if (primitive.isString()) {
         return new StringValue(primitive.getAsString());
       }
     }
-    else if (node.isJsonNull()) {
-      return null;
-    }
     throw new IOException("Cannot convert value: unsupported JSON value " + node.toString());
   }