Report: disallow saving and printing when report is empty

[jerneju]

Orange version

current (3.5.dev)

Expected behavior

Report widget should not allow a user to save or print an "empty" report. Save and Print buttons should be disabled.

Actual behavior

One can save or print empty report. It can be saved in a html file where only some CSS code is placed. That is meaningless.

Steps to reproduce the behavior

See above.

Additional info (worksheets, data, screenshots, ...)