[processing] make additional Fusion parameters optional

Author: alexbruy

python/plugins/processing/algs/lidar/fusion/FusionAlgorithm.py

@@ -51,7 +51,7 @@ def checkBeforeOpeningParametersDialog(self):
 
     def addAdvancedModifiers(self):
         param = ParameterString(
-            self.ADVANCED_MODIFIERS, self.tr('Additional modifiers'), '')
+            self.ADVANCED_MODIFIERS, self.tr('Additional modifiers'), '', optional=True)
         param.isAdvanced = True
         self.addParameter(param)