[ADF-621] show drop effect on folders only (#1897)

* show drop effect on folders only

- fix `hasValue` api for data rows (avoid 'false' value to be evaluated as missing value)
- support for evaluating drop support for rows
- document list enables upload zones for folders only

* api improvements as per code review

ng2-components/ng2-alfresco-datatable/src/data/object-datatable-adapter.ts

@@ -209,7 +209,7 @@ export class ObjectDataRow implements DataRow {
     }
 
     hasValue(key: string): boolean {
-        return this.getValue(key) ? true : false;
+        return this.getValue(key) !== undefined;
     }
 }