fix bool query when the body is empty (#263)

src/Query/Compound/BoolQuery.php

@@ -116,6 +116,10 @@ class BoolQuery implements BuilderInterface
 
         $output = $this->processArray($output);
 
+        if (empty($output)) {
+            $output = new \stdClass();
+        }
+
         return [$this->getType() => $output];
     }
 

tests/Unit/Query/Compound/BoolQueryTest.php

@@ -69,6 +69,16 @@ class BoolQueryTest extends \PHPUnit_Framework_TestCase
         $this->assertEquals($expected, $bool->toArray());
     }
 
+    /**
+     * Tests bool query with empty body if it forms \stdObject
+     */
+    public function testEmptyBoolQuery()
+    {
+        $bool = new BoolQuery();
+
+        $this->assertEquals(['bool' => new \stdClass()], $bool->toArray());
+    }
+
     /**
      * Tests bool query in filter context.
      */