Do not fail a call to paypal or stripe with no return data.
810643ceb6ee
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Description

Do not fail a call to paypal or stripe with no return data.

* src/paypal.c (call_paypal): Replace an empty string by a JSON
object.
* src/stripe.c (call_stripe): Ditto.

Some Paypal call s do not return data. This results in an empty
string in our functions which is a non-valid JSON object. The effect
is that an error is returned and not an "empty" JSON object. To
mitigate this we return the "null" JSON object in such a case.