zcldec: AttrSpec should return null for missing attributes

Previously it was returning DynamicVal, but that's incorrect since it
would mean that even an otherwise-complete result that has an unpopulated
optional attribute would include an unknown.

zcldec/public_test.go

@@ -64,7 +64,7 @@ func TestDecode(t *testing.T) {
 				Required: true,
 			},
 			nil,
-			cty.DynamicVal,
+			cty.NullVal(cty.Number),
 			1, // attribute "a" is required
 		},
 

zcldec/spec.go

@@ -165,7 +165,7 @@ func (s *AttrSpec) decode(content *zcl.BodyContent, block *zcl.Block, ctx *zcl.E
 	if !exists {
 		// We don't need to check required and emit a diagnostic here, because
 		// that would already have happened when building "content".
-		return cty.DynamicVal, nil
+		return cty.NullVal(s.Type), nil
 	}
 
 	// TODO: Also try to convert the result value to s.Type