Fix KDevPluginController::query() with empty constraint

When KDevPluginController::query() was called with empty constrain it
were resulting in incorrect query string being passed to TDETrader,
which were resulting in at least warnings like the next one:

```
[tdeio (TDETrader)] [6751] WARNING: Parsing ' and [X-TDevelop-Version] == 5' gave syntax error
```

Signed-off-by: Alexander Golubev <fatzer2@gmail.com>

lib/interfaces/kdevplugincontroller.cpp

@@ -27,8 +27,17 @@ KDevPluginController::KDevPluginController()
 TDETrader::OfferList KDevPluginController::query(const TQString &serviceType,
     const TQString &constraint)
 {
-    return TDETrader::self()->query(serviceType,
-        TQString("%1 and [X-TDevelop-Version] == %2").arg(constraint).arg(TDEVELOP_PLUGIN_VERSION));
+    TQString versionConstraint =
+        TQString::fromLatin1("[X-TDevelop-Version] == %1").arg(TDEVELOP_PLUGIN_VERSION);
+
+    TQString newConstraint;
+    if (!constraint.isEmpty()) {
+        newConstraint = TQString("( %1 ) and %2").arg(constraint).arg(versionConstraint);
+    } else {
+        newConstraint = versionConstraint;
+    }
+
+    return TDETrader::self()->query(serviceType, newConstraint);
 }
 
 TDETrader::OfferList KDevPluginController::queryPlugins(const TQString &constraint)