Provide compatibility to asset links

Scenario:
Due to MGNLUI-5840, asset references have been stored without the prefix jcr:. When opening their corresponding dialog, which is declared with the old syntax, these references are throwing an error like the following:

java.lang.IllegalArgumentException: 507b0635-d1b1-49ab-99d4-451751e0fd08 is not a valid ItemKey

Removing targetWorkspace property from the definition makes it work ok but, unfortunately, preview is not shown as reported in MGNLUI-5979.
Steps to reproduce (Magnolia 6.2.3)

1. Create a component whose dialog contain following field:

   - name: cinemagraph
             class: info.magnolia.ui.form.field.definition.LinkFieldDefinition
             required: true
             targetWorkspace: dam
             appName: assets
             identifierToPathConverter:
               class: info.magnolia.ui.form.field.converter.BaseIdentifierToPathConverter
             contentPreviewDefinition:
               contentPreviewClass: info.magnolia.dam.app.ui.field.DamFilePreviewComponent 

2. Store any link by using the dialog (Note the preview is shown in the dialog) and remove the jcr: prefix from the reference through JCR app (Forcing MGNLUI-5840)
3. Open the component dialog again and see the error.
4. Remove targetWorkspace property from the definition and open the component dialog again. It works ok and no error is thrown now but preview is not shown in the dialog

Expected results
There should be compatibility for assets that were stored without jcr: prefix.