Bug report: Pad Lines outputs [object Object] with empty length field

**Describe the bug**
Pad Lines operation with empty Length field outputs `[object Object]`

**To Reproduce**
Steps to reproduce the behaviour or a link to the recipe / input used to cause the bug:

- Use "Pad Lines" operation
- Set Length field to be empty

https://gchq.github.io/CyberChef/#recipe=Pad_lines('Start',null,'test%20')&input=aGk

**Expected behaviour**
Either output an actual error if length is not set, or use some sensible default behavior.
In my opinion, a useful default behavior would be to assume Length to be the length of the "Character" textbox input. This way, if you want to pad a line with some string, you don't have to manually compute the string length to use Pad Lines

**Desktop (if relevant, please complete the following information):**
- Browser: Chrome 134
- CyberChef version: 10.19.4