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.
For rationale, see #6787
Extensions migrated in part 4:
* simplexml
* skeleton
* soap
* spl
* sqlite3
* sysvmsg
* sysvsem
* tidy - also removed a check for an ancient dependency version
