Custom plugins defined as default parameters throw NPE when relying on request args that are omitted

[varontron]

Problem:
Custom plugin defined as default query parameter which expects arguments to be passed in URL throws NPE when arguments are omitted.

Steps to repro:
Design the custom plugin to take get called at the query level, but to use args passed in the request parameter
Deploy the custom plugin in a local YADA instance
Define a default parameter on a query to call the plugin
Omit the pl=plugin,args parameter from the request

Result: NPE