increment once at the start of the loop

Author: jonathan343

codegen/core/src/main/java/software/amazon/smithy/python/codegen/generators/StructureGenerator.java

@@ -421,18 +421,18 @@ def _consumer(schema: Schema, de: ShapeDeserializer) -> None:
     }
 
     private void deserializeMembers(Collection<MemberShape> members) {
-        int index = 0;
+        int index = -1;
         for (MemberShape member : members) {
+            index++;
             var target = model.expectShape(member.getTarget());
             if (target.hasTrait(StreamingTrait.class) && target.isUnionShape()) {
-                index++;
                 continue;
             }
             writer.write("""
                     case $L:
                         kwargs[$S] = ${C|}
                     """,
-                    index++,
+                    index,
                     symbolProvider.toMemberName(member),
                     writer.consumer(
                             w -> target.accept(new MemberDeserializerGenerator(context, writer, member, "de"))));