if-has-value

The if-has-value Instruction conditionally executes a nested lists of Instructions depending on whether a Destination has a value set. This is useful for setting default values.

`action`	`"if-has-value"`
`destination`	The Destination to check.
`name`	The Name within the Destination.
`then`	An array of Instructions to be executed when the value exists in the Destination (optional).
`else`	An array of Instructions to be executed when the value does not exist in the Destination (optional).

Example: setting defaults

To ensure a object always has a title by applying a default if no other value has been set, add this to the end of your control file’s Instructions.

{
    "action": "if-has-value",
    "destination": "book",
    "name": "dc:attribute:title",
    "else": [
        {
            "action": "set-value",
            "destination": "book",
            "name": "dc:attribute:title",
            "value": "Unnamed project"
        }
    ]
}