[processing] Correct management of optional rasters in batch mode. Fixes

 #19115

python/plugins/processing/gui/BatchInputSelectionPanel.py

@@ -183,7 +183,7 @@ def textEditingFinished(self):
         self.valueChanged.emit()
 
     def value(self):
-        return self._value
+        return self._value if self._value else None
 
     def setValue(self, value):
         self._value = value