Add support for the default keyword in pass-arguments-to-command.
When provided, this value is used if the user does not supply the argument or if it is empty, instead of raising an error or using an empty value.
Documentation updated in docs/Referencing-Request-Values.md.
Example:

{
  "source": "url",
  "name": "q",
  "default": "123"
}

In this example, if the URL query does not contain q or it is empty, the value "123" will be used.
If default is omitted, the behavior remains unchanged.