fix(medusa-js): Fix stringifyNullProperties util (#1766)

**What**
Changes the order of object type evaluation to properly handle null properties.

Previously, `stringifyNullProperties({ test: null })` would fail in the 2nd iteration due to an attempt to iterate `null` in `Object.keys(obj)`.

packages/medusa-js/test/utils/utils.test.ts

@@ -0,0 +1,31 @@
+import { stringifyNullProperties } from "../../src/utils"
+
+describe("stringifyNullProperties", () => {
+  test("returns empty object on no props", () => {
+    const result = stringifyNullProperties({})
+    expect(result).toEqual({})
+  })
+
+  test("successfully stringifies null property", () => {
+    const result = stringifyNullProperties({ test: null })
+    expect(result).toEqual({ test: "null" })
+  })
+
+  test("successfully stringifies nested null property", () => {
+    const result = stringifyNullProperties({
+      test: { test_2: { test_3: null } },
+      another_test: "test",
+    })
+    expect(result).toEqual({
+      test: { test_2: { test_3: "null" } },
+      another_test: "test",
+    })
+  })
+
+  test("successfully stringifies string property", () => {
+    const result = stringifyNullProperties({
+      test: "test",
+    })
+    expect(result).toEqual({ test: "test" })
+  })
+})