Fix preview when a document type does not have the file schema

ERROR [http-bio-0.0.0.0-8080-exec-15] [org.nuxeo.ecm.platform.actions.DefaultActionFilter] evaluation of condition #{previewActions.hasPreview} failed: returning false
javax.el.ELException: Error reading 'hasPreview' on type org.nuxeo.ecm.platform.preview.seam.PreviewActionBean_$$_javassist_seam_56
...
Caused by: org.nuxeo.ecm.core.api.model.PropertyNotFoundException: segment 0 cannot be resolved, files:files/0/file
	at org.nuxeo.ecm.core.api.impl.DocumentModelImpl.getProperty(DocumentModelImpl.java:1374)
	at org.nuxeo.ecm.core.api.impl.DocumentModelImpl.getPropertyValue(DocumentModelImpl.java:1424)
	at org.nuxeo.ecm.core.api.blobholder.DocumentBlobHolder.getBlob(DocumentBlobHolder.java:61)
	at org.nuxeo.ecm.platform.preview.adapter.base.ConverterBasedHtmlPreviewAdapter.getBlob2preview(ConverterBasedHtmlPreviewAdapter.java:133)
	at org.nuxeo.ecm.platform.preview.adapter.base.ConverterBasedHtmlPreviewAdapter.hasBlobToPreview(ConverterBasedHtmlPreviewAdapter.java:201)
	at org.nuxeo.ecm.platform.preview.helper.PreviewHelper.docHasBlobToPreview(PreviewHelper.java:109)
	at org.nuxeo.ecm.platform.preview.seam.PreviewActionBean.documentHasPreview(PreviewActionBean.java:102)
	at org.nuxeo.ecm.platform.preview.seam.PreviewActionBean.getHasPreview(PreviewActionBean.java:93)
...