incorrect MultiTooltip appearance with empty title

[dima117]

As far as i understand, MultiTooltip converts the "title" property to string and displays it. In this case appearance of tooltip title is incorrect, if "title" property is not specified.
If title is null then displayed "null". If title is nudefined then displayed "undefined". And if title is empty string then excess top padding is displayed.

If the title is not specified, it is not necessary to show and to provide a place for it

[etimberg]

Closing as #1040 was merged