fix: revert the mapTransform test to previous implementation

Author: oscb

packages/core/src/__tests__/__snapshots__/mapTransform.test.ts.snap

@@ -14,14 +14,6 @@ Object {
       "doNotTransform": "nope",
       "muchNewerKey": "123",
     },
-    Object {
-      "doNotTransform": "nope",
-      "muchNewerKey": "123",
-    },
-    Object {
-      "doNotTransform": "nope",
-      "muchNewerKey": "123",
-    },
   ],
   "newId": 1,
 }

packages/core/src/__tests__/mapTransform.test.ts

@@ -16,8 +16,8 @@ describe('mapTransform', () => {
       return value;
     },
     newArray: (value: unknown) => {
-      if (isString(value)) {
-        return value.slice(0, 1);
+      if (Array.isArray(value)) {
+        return value.slice(0, 1) as unknown[];
       }
       return value;
     },