Changes

[508435] Accept null (displayed as empty string) as a valid text value

If the callback was called with null as the new text value, we did not
refresh the UI at all, leaving the previous text visible. It is
perfectly valid for a text value to be null in a model, for example if
it represents an unset string attribute.

Bug: 508435
Change-Id: I6ec17a36028e2d39debf4d0a6e82e40d4a6a9f42
Signed-off-by: Pierre-Charles David <pierre-charles.david@obeo.fr>