Fix QPaintDevice metrics warnings for DevicePixelRatio metrics

Handle PdmDevicePixelRatio and PdmDevicePixelRatioScaled metrics in
QwtNullPaintDevice.metric() to prevent 'Device has no metric information'
warnings when falling back to super().metric().

This fixes warnings introduced by commit b86f058 which changed the default
case from returning 0 to calling super().metric() for unhandled metrics.

Author: PierreRaybaut

qwt/null_paintdevice.py

@@ -272,6 +272,10 @@ def metric(self, deviceMetric):
                 * 25.4
                 / self.metric(QPaintDevice.PdmDpiY)
             )
+        elif deviceMetric == QPaintDevice.PdmDevicePixelRatio:
+            value = 1
+        elif deviceMetric == QPaintDevice.PdmDevicePixelRatioScaled:
+            value = 1
         else:
             value = super(QwtNullPaintDevice, self).metric(deviceMetric)
         return value