Fixed empty param for v2 requests

[prathmesh-stripe]

Creating a meter event session was failing(400) because we were trying to send an invalid json body in the request.

$stripeClient->v2->billing->meterEventSession->create([]);

json_encode([]) // '[]' - string brackets
json_encode(null) // 'null' - String null

In order to fix this, I added a check to skip encoding params when the params are empty.