FCPPluginReply: Add asserts() to constructor to specify requirements of the parameters
- Use assert instead of throwing so the deprecated constructor which uses this one does not break outside of development environments. Also, this function will be used a lot, so it would suck to have null-checks execute always.
- The SFS can be null: getFieldSet() uses SimpleFieldSet.put(String, SimpleFieldSet) to add it to another SimpleFieldset, and that put() allows it to be null.