[processing] minor change in ParameterFile to handle None values

Author: volaya

python/plugins/processing/parameters/ParameterFile.py

@@ -46,9 +46,11 @@ def serialize(self):
 
     def setValue(self, obj):
         self.value = str(obj)
-        if self.value.strip() == '':
+        if self.value.strip() == '' or self.value is None:
             if not self.optional:
                 return False
+            else:
+                self.value = ''
         return True
 
     def deserialize(self, s):