Check if a string is null or empty and return a Visibility value. if the string is null or empty, return Collapsed, otherwise return Visible.
...
<ToolTipService.ToolTip>
<ToolTip Content = "{x:Bind ButtonToolTip, Mode=OneWay}" Visibility="{x:Bind ButtonToolTip, Converter={StaticResource StringToVisibilityConverter}, Mode=OneWay}" />
</ToolTipService.ToolTip>