perf(ngModel,form): avoid invoking $interpolate for empty/unset values

This reduces GC (KB) by ~10% in the added benchmark create step.

I'm tempted to add a helper such as `$interpolate.$$eval(exp, scope)` to avoid the ugly duplication. A helper could also be used for spots such as https://github.com/angular/angular.js/blob/master/src/ng/compile.js#L1922 (the helper can pass `mustHaveExpression=true` to avoid creating the interp result function even though it has no interpolation...). Worth it?