Handle case where no results are returned

When 0 results are returned, Forage returns a string "no results"
instead of a properly formatted JSON string. So json_decode() returns
null, and that gets passed to SearchResult. SearchResult only accepts an
array as input, and throws an E_RECOVERABLE_ERROR. Http Transport now
intercepts this case, and outputs a properly formatted array containing
no hits or facets, and set totalHits to 0.

PHP error:
Catchable fatal error: Argument 1 passed to
vierbergenlars\Forage\SearchResult\SearchResult::__construct() must be
an array, null given, called in
/var/www/bay/vendor/vierbergenlars/forage-client/lib/vierbergenlars/Forage/SearchQuery/Query.php
on line 174 and defined in
/var/www/bay/vendor/vierbergenlars/forage-client/lib/vierbergenlars/Forage/SearchResult/SearchResult.php
on line 42

XDebug Trace:
8	0.1450	21947952
vierbergenlars\Forage\ODM\SearchQuery->execute( )
../SearchController.php:43
9	0.1451	21947952
vierbergenlars\Forage\SearchQuery\Query->execute( )
../SearchQuery.php:44
10	0.1490	21999232
vierbergenlars\Forage\SearchResult\SearchResult->__construct(
$result_array = NULL )	../Query.php:174