asMapOrNull<T, S> function
==================== Map conversions
Returns a Map if the value at key is a map, otherwise null.
Implementation
/// Returns a Map if the value at [key] is a map, otherwise null.
Map<T, S>? asMapOrNull<T, S>(dynamic json, String key) {
final value = _getJsonValueOrNull(json, key);
return toMapOrNull(value);
}