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 'PHP-5.4' into PHP-5.5

* PHP-5.4:
  Fix #66562: Consistency bug where curl_multi_getcontent behaves different from curl_exec