writer/msl: Simplify type constructor generation

The special-case for zero-valued constructors is unnecessary, as an
empty initializer list already correctly zero-initializes for all
types. This was causing an additional {} to be emitted for empty
structures, which the MSL compiler rejects.

Fixed: tint:821
Change-Id: Ib48c73eadef15b517e14b248229ecfbbfeb13f81
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/51822
Commit-Queue: James Price <jrprice@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Auto-Submit: James Price <jrprice@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>

test/samples/simple.wgsl.expected.msl

@@ -9,7 +9,7 @@ void bar() {
 }
 
 fragment tint_symbol_1 tint_symbol() {
-  float2 a = float2(0.0f);
+  float2 a = float2();
   bar();
   return {float4(0.400000006f, 0.400000006f, 0.800000012f, 1.0f)};
 }