fix: add is_object check

[ishetnogferre]

Importing via Feed me throws following error: An exception has been thrown during the rendering of a template ("property_exists(): Argument #1 ($object_or_class) must be of type object|string, array given") on lines where the property_exists function is used.

Changes proposed in this pull request:

• Double-check if parts is an object

Note: this is a quick fix we did to make it work on a project, there might other fixes needed in the FeedMe mapper