Handle TM and MT merge modal when there is no document in the version.

If there is no document in the version still MT and TM merge modal open and shows some action/progress

Steps to Reproduce:

• Login to zanata

• Go to dashboard

• Create project and version

• Do not upload any document in version

• Run MT and TM merge from right top options

• Observe actions

Actual Result:

• TM and MT merge modal opens and shows some progress on run even if there is no any document

Expected result:

• MT and TM merge modal should not open if there is no any document

• it should show a notification that no documents in version to run MT/TM