View Designer: fix badly generated nuxeo-dropzone for workflow task layout

Steps to reproduce:

• In the modeler part, define a workflow model which has a multi blob variable name myBlobs
• add a validate/reject node
• in the variables tab this new node, add the myBlobs in the Workflow Variables Editable in this Task (see attached screenshot1.png)
• In the designer part, generate the task layout for this new node (see screenshot2.png)

The generated widget for the blob list var is:

<nuxeo-dropzone document="{{document}}" xpath="task:blobs" blob-list="true" blob-append="true"></nuxeo-dropzone>

which is wrong:

• nuxeo-dropzone has been designed to work with a document
• we are in the ctx of a workflow processing, there's no such document available

NOTE that in case of a single blob variable, the following is generated:

<nuxeo-file value="{{task.variables.myBlob}}"></nuxeo-file>

which is correct.