The do_request() function that calls this methods, assumes that a string is being returned from the method
otherwise it bails out.
However, the default implementation of SoapClient::__doRequest() indicates that it can return null when it
fails to set-up and execute the HTTP SOAP request, but this always results in a SoapFault exception being
thrown, and thus cannot happen in practice.
We need to investigate further if the return type should be changed from ?string to string or not.
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