Allow null indexes on formSerializer and paramsSerializer (#4960)

* test: Failed test

Param indexes from formSerializer and paramsSerializer receiving null

Closes #4959

* fix: Allow null to indexes in SerializerOptions

Closes #4959

Co-authored-by: Willian Agostini <willian.agostini@fleetcor.com.br>
Co-authored-by: Jay <jasonsaayman@gmail.com>

test/typescript/axios.ts

@@ -269,6 +269,17 @@ instance1.defaults.timeout = 2500;
 
 axios.create({ headers: { foo: 'bar' } });
 axios.create({ headers: { common: { foo: 'bar' } } });
+axios.create({
+  headers: {
+    'Content-Type': 'application/json',
+  },
+  formSerializer: {
+    indexes: null,
+  },
+  paramsSerializer: {
+    indexes: null,
+  },
+});
 
 // Interceptors