When a tooltip isn't desired (setting column-definition to empty stri…

[TimoLehtinen70]

…ng), there will be console error "Text for the tooltip has not been provided. Please include text within the mdTooltip element.".

This change will fix it by not including tooltip into DOM when there's no text.

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This change is 

[lenin-anzen]

I am getting the same error, with this PR the error is gone...