[!!!][BUGFIX] Correct object modification exception trigger
The PersistentObjectConverter throws an exception if there are
properties to be set on the object and modification was not allowed
in the PropertyMappingConfiguration.
The decision if there are properties to be set was done based on the
amount of entries in the ``$source`` array, but in fact only the
``$convertedChildProperties`` are set to the model, so the check
should check if there is anything in that array.
That means you can have any amount of arbitrary data in your data
source as long as it is not converted to an actual child property.
Which is determined by the ``getSourceChildPropertiesToBeConverted``
method of the converter.
This is breaking if you rely on the fact that the exception is thrown
if you have arbitrary data in $source even though that data would
never have been set to your model.