Add check for null value in Option form field

Missing check resulted in critical error in attempts for Variant
manipulation. Null value means unchecked state of the option entry.

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Separate id and target in Option form entry

Previously id and target were specified in a single string "id:target",
now they are separate strings, so API users don't need to concat
the strings:

var ADDRESS = "app.address";
var ADDRESS_DEFAULT = "default";

entries.push(["option", ADDRESS, ADDRESS_DEFAULT, "Default"]);

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>