ECMS-4550: Uploading a file into Web Contents folder breaks the WCM Content Selector

Fix description:

   - check if the property of exo:dateCreated exists before using
ECMS-4550: Uploading a file into Web Contents folder breaks the WCM Content Selector

Fix description:
    - Fix to not display null value in date field.
Merge branch 'fix/2.3.x/ECMS-4550' into stable/2.3.x