nuxeo-file element nullifies the values if no edit in a workflow

Following to ELEMENTS-748, it is now possible to attach a file to a workflow.
It is possible to read the file in a following task.

But it does not work if more than 2 tasks.

For instance, the following 3 tasks scenario shows the issue:

1. create a workflow with 3 tasks
2. add a workflow variable
3. make it available and editable in each of them
4. set assignee to administrators group
5. start nuxeo
6. sync the project
7. create a document
8. start the workflow
9. attach a file (task 1)
10. approve the task
11. observe the following traffic in Google Inspector:

    2: {entity-type: "task", id: "b1aae882-1aae-435e-9be7-d4940cc2b751", name: "ApproveB",…}
    actors: [{entity-type: "group", groupname: "administrators", grouplabel: "Administrators group",…}]
    comments: []
    created: "2019-03-18T10:41:40.618Z"
    delegatedActors: []
    directive: "Acknowledgement"
    dueDate: "2019-03-18T10:41:40.615Z"
    entity-type: "task"
    graphResource: "http://localhost:8080/nuxeo/api/v1/workflow/ef24afbe-ce44-49da-ad9b-37f734b82f82/graph"
    id: "b1aae882-1aae-435e-9be7-d4940cc2b751"
    name: "ApproveB"
    nodeName: "testWkfBlob-20190318111151123-approve-task"
    state: "opened"
    targetDocumentIds: [{entity-type: "document", repository: "default", uid: "26b7ae70-01c0-4cc0-a8a2-1d8f6eae5397",…}]
    taskInfo: {allowTaskReassignment: false, taskActions: [{name: "approve",…}],…}
    variables: {myBlob: {name: "algo_webui.txt", mime-type: "text/plain", encoding: null, digestAlgorithm: "MD5",…}}
    myBlob: {name: "algo_webui.txt", mime-type: "text/plain", encoding: null, digestAlgorithm: "MD5",…}
    data: "http://localhost:8080/nuxeo/nxfile/default/ef24afbe-ce44-49da-ad9b-37f734b82f82/var_testWkfBlob:myBlob/algo_webui.txt"
    digest: "94818c31fce8fef9e83fb0a7ff939d2c"
    digestAlgorithm: "MD5"
    encoding: null
    length: "623"
    mime-type: "text/plain"
    name: "algo_webui.txt"
    workflowInitiator: "Administrator"
    workflowInstanceId: "ef24afbe-ce44-49da-ad9b-37f734b82f82"
    workflowLifeCycleState: "running"
    workflowModelName: "testWkfBlob"
    workflowTitle: "testWkfBlob"
    

12. in task 2, observe the file if correctly displayed
13. approve the task
14. observe the following traffic in Google Inspector:

    2: {entity-type: "task", id: "ffbc1b73-02a5-4967-a750-40ab0c260148", name: "ApproveC",…}
    actors: [{entity-type: "group", groupname: "administrators", grouplabel: "Administrators group",…}]
    comments: []
    created: "2019-03-18T10:44:59.500Z"
    delegatedActors: []
    directive: "Acknowledgement"
    dueDate: "2019-03-18T10:44:59.498Z"
    entity-type: "task"
    graphResource: "http://localhost:8080/nuxeo/api/v1/workflow/ef24afbe-ce44-49da-ad9b-37f734b82f82/graph"
    id: "ffbc1b73-02a5-4967-a750-40ab0c260148"
    name: "ApproveC"
    nodeName: "testWkfBlob-20190318112514074-approve-task"
    state: "opened"
    targetDocumentIds: [{entity-type: "document", repository: "default", uid: "26b7ae70-01c0-4cc0-a8a2-1d8f6eae5397",…}]
    taskInfo: {allowTaskReassignment: false, taskActions: [{name: "approve",…}],…}
    variables: {myBlob: null}
    myBlob: null
    workflowInitiator: "Administrator"
    workflowInstanceId: "ef24afbe-ce44-49da-ad9b-37f734b82f82"
    workflowLifeCycleState: "running"
    workflowModelName: "testWkfBlob"
    workflowTitle: "testWkfBlob"
    

15. observe the myBlob variable value is now null
16. in task 3, observe the attached file does not appear any longer