COMP: Fix build error in ctkLDAPExpr.cpp

Fix typo that was made recently when fixing deprecated QString::null.

From Qt changes-5.9.0 document:
QString::null is now deprecated. When used to construct a QString, use
QString() instead. When used to compare to a QString, replace with
QString::isNull().

fixes #925

Libs/PluginFramework/ctkLDAPExpr.cpp

@@ -620,7 +620,7 @@ QString ctkLDAPExpr::ParseState::getAttributeName()
     }
   }
   if (n == -1) {
-    return QString()( );
+    return QString();
   }
   return m_str.mid(start, n);
 }