This Cl updates the system to allow zero initializers. This allows:
```
var a : vec3<f32> = vec3<f32>();
```
Bug: tint:34
Change-Id: I84d6b431914c4ddf112ed375fae028d912f4a080
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/23660
Reviewed-by: David Neto <dneto@google.com>