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/src/utils.ts

@@ -3,12 +3,10 @@ export function stringifyNullProperties<T extends object>(input: T): T {
     const res = {} as T
 
     Object.keys(obj).reduce((acc: T, key: string) => {
-      if (typeof obj[key] === "object") {
-        acc[key] = convertProperties(obj[key])
-      }
-
       if (obj[key] === null) {
         acc[key] = "null"
+      } else if (typeof obj[key] === "object") {
+        acc[key] = convertProperties(obj[key])
       } else {
         acc[key] = obj[key]
       }