object / data types: bug fixes in combination with Object_Class_Data::isEmpty(): isEmpty() should only be used with real data values not with editmode specific data, isEmpty() should be used consistently across pimcore

git-svn-id: http://www.pimcore.org/svn/pimcore/private/core/trunk@5315 1f8fe7d8-47f0-464c-8d0a-336f4953ab05