将0值格式化为空字符串?

I'm struggling with my first foray into WPF string formatting. I'd like to be able to format a textbox column in a data grid with an empty string when the underlying value is zero and format all other values as 0.000. However, my XAML doesn't seem to be up to the job as it shows blanks for I'm struggling with my first foray into WPF str