fix(core/schema): allow structiterator call on unit schema

.changeset/serious-waves-cheat.md

@@ -0,0 +1,5 @@
+---
+"@smithy/core": patch
+---
+
+allow struct iterator acquisition on unit schema

packages/core/src/submodules/schema/schemas/NormalizedSchema.spec.ts

@@ -185,6 +185,11 @@ describe(NormalizedSchema.name, () => {
         expect(schema.getMergedTraits()).toEqual(entrySchema.getMergedTraits());
       }
     });
+
+    it("can acquire structIterator on the unit schema type and its iteration is empty", () => {
+      const iteration = Array.from(NormalizedSchema.of("unit").structIterator());
+      expect(iteration.length).toBe(0);
+    });
   });
 
   describe("traits", () => {

packages/core/src/submodules/schema/schemas/NormalizedSchema.ts

@@ -401,6 +401,9 @@ export class NormalizedSchema implements INormalizedSchema {
    * This avoids the overhead of calling Object.entries(ns.getMemberSchemas()).
    */
   public *structIterator(): Generator<[string, NormalizedSchema], undefined, undefined> {
+    if (this.isUnitSchema()) {
+      return;
+    }
     if (!this.isStructSchema()) {
       throw new Error("@smithy/core/schema - cannot acquire structIterator on non-struct schema.");
     }