Make computeNbResult() compatible with GROUP BY

Using getOneOrNullResult() causes an exception in case of a query with
GROUP BY. To fix the issue, we allow multiple rows and do a sum over the
cnt column, which holds the count for each group.

Fixes sonata-project#968

src/Datagrid/Pager.php

@@ -45,7 +45,10 @@ public function computeNbResult()
             current($this->getCountColumn())
         ));
 
-        return (int) ($countQuery->resetDQLPart('orderBy')->getQuery()->getOneOrNullResult(Query::HYDRATE_SINGLE_SCALAR));
+        return array_sum(array_column(
+            $countQuery->resetDQLPart('orderBy')->getQuery()->getResult(Query::HYDRATE_SCALAR),
+            'cnt'
+        ));
     }
 
     public function getResults($hydrationMode = Query::HYDRATE_OBJECT)

tests/Datagrid/PagerTest.php

@@ -62,11 +62,12 @@ public function testComputeNbResult($distinct): void
     {
         $query = $this->getMockBuilder(AbstractQuery::class)
             ->disableOriginalConstructor()
-            ->setMethods(['getOneOrNullResult'])
+            ->setMethods(['getResult'])
             ->getMockForAbstractClass();
 
         $query->expects($this->once())
-            ->method('getOneOrNullResult');
+            ->method('getResult')
+            ->willReturn([['cnt' => 1], ['cnt' => 2]]);
 
         $queryBuilder = $this->getMockBuilder(QueryBuilder::class)
             ->disableOriginalConstructor()