string static method
Safely converts a dynamic value v to String.
Returns def if the value is null.
If the value is not a String, it calls .toString().
Example:
JsonCare.string(123); // "123"
JsonCare.string(null, def: "N/A"); // "N/A"
Implementation
static String string(dynamic v, {String def = ""}) {
if (v == null) return def;
if (v is String) return v;
_logMismatchedType("String", v);
return v.toString();
}