Fix #66562: Consistency bug where curl_multi_getcontent behaves different from curl_exec

curl_exec returns an empty string when data is received from a domain that returns zero content. curl_multi_getcontent
returned null. Now it returns an empty string fixing the incosistency
Merge branch 'pull-request/578'

* pull-request/578:
  Fix #66562: Consistency bug where curl_multi_getcontent behaves different from curl_exec