Regression of ZLP sending

[uwni]

Hi, I noticed that MicroZig could send the ZLP properly in CDC in the earlier version. But after this PR, It was removed. Is it intended?

[mattnite]

@piotrfila might be able to give context, otherwise it sounds like a regression.

[piotrfila]

I did that commit before I fully understood the spec, I thought I had added it back but it might have been on a branch I never merged back