[processing] Set value of unused optional layer parameters to None instead of empty string

Author: volaya

python/plugins/processing/gui/wrappers.py

@@ -795,7 +795,7 @@ def value(self):
                 if layer is not None:
                     return layer
                 else:
-                    return self.combo.currentText()
+                    return self.combo.currentText() or None
             except:
                 return self.combo.currentText()
         elif self.dialogType == DIALOG_BATCH: