set nodata parameter as optional in GDAL clip by extent

python/plugins/processing/algs/gdal/ClipByExtent.py

@@ -64,7 +64,7 @@ def defineCharacteristics(self):
             self.INPUT, self.tr('Input layer'), False))
         self.addParameter(ParameterString(self.NO_DATA,
                                           self.tr("Nodata value, leave blank to take the nodata value from input"),
-                                          ''))
+                                          '', optional=True))
         self.addParameter(ParameterExtent(self.PROJWIN, self.tr('Clipping extent')))
 
         params = []