fix: correctly decode falsy values in legacy protocol buffers

[chrisrossi]

Fixes #625

[chrisrossi]

This is a pretty good object lesson on why the old expr and truthy_value or falsy_value way of faking a ternary operator is a pretty bad construct. What you want is to get truthy_value or falsy_value based only on the evaluation of expr, but in this case, if truthy_value is falsy you get falsy_value regardless of how expr evaluates.

Converting to the native Python ternary operator construct fixes this.