Fix U4-2711 add Null Check to ToXMl method

Fixes Object reference not set to an instance of an object. at
umbraco.editorControls.imagecropper.DataTypeData.ToXMl(XmlDocument data)
Merge branch 'U4-2711-Fix-imagecropper-ToXMl' of https://github.com/stocksr/Umbraco-CMS into stocksr-U4-2711-Fix-imagecropper-ToXMl

Conflicts:
	src/umbraco.editorControls/imagecropper/DataTypeData.cs