Fix DOMDocument Empty String PHP Warning

- Fix DOMDocument::loadHTML() throwing a PHP Warning if the result of
  the cURL call is an empty string. Checks that the result of the curl
  call isn't empty before passing it to _parse() function.
Merge pull request #10 from MitchellMcKenna/feature/DOMDocument-loadHTML-empty-string-warning

Fix DOMDocument Empty String PHP Warning