don't create field when not exist

Author: faissaloux

src/Query/Builder.php

@@ -907,9 +907,9 @@ public function multiply($column, $amount, array $extra = [], array $options = [
 
         // Protect
         $this->where(function ($query) use ($column) {
-            $query->where($column, 'exists', false);
+            $query->where($column, 'exists', true);
 
-            $query->orWhereNotNull($column);
+            $query->whereNotNull($column);
         });
 
         $options = $this->inheritConnectionOptions($options);

tests/QueryBuilderTest.php

@@ -1098,7 +1098,7 @@ public function testMultiplyAndDivide()
         $user = DB::table('users')->where('name', 'Robert Roe')->first();
         $this->assertNull($user->salary);
         $user = DB::table('users')->where('name', 'Mark Moe')->first();
-        $this->assertEquals(0, $user->salary);
+        $this->assertFalse(isset($user->salary));
     }
 
     public function testProjections()