Fix issue in remove-undefined-objects with null that would cause it to throw

packages/api-explorer/__tests__/lib/remove-undefined-objects.test.js

@@ -7,3 +7,7 @@ test('should remove empty objects with only undefined properties', () => {
 test('should not throw on arrays of primitives', () => {
   expect(removeUndefinedObjects([null])).toEqual([null]);
 });
+
+test('should not throw for null', () => {
+  expect(removeUndefinedObjects({ a: null })).toEqual({ a: null });
+});

packages/api-explorer/src/lib/remove-undefined-objects.js

@@ -7,7 +7,7 @@ function isEmptyObject(obj) {
 function stripEmptyObjects(obj) {
   Object.keys(obj).forEach(key => {
     const value = obj[key];
-    if (typeof value === 'object' && !Array.isArray(obj)) {
+    if (typeof value === 'object' && !Array.isArray(obj) && value !== null) {
       // Recurse, strip out empty objects from children
       stripEmptyObjects(value);
       // Then remove all empty objects from the top level object