Fix command-line arguments to use empy string as default for flag.Parse

* This will allow for test files to be run without panicking
* If -file is not specified, we still use the first argument provided
* If no command-line argument is provided, panic
Update gojson to use "*interface{}" whenever JSON value is null

* We can't infer the actual type, but we know it must be nullable
* Therefore, a pointer (which can be nil) to interface{} is easiest