If you choose to reset all, that is what Lightroom will do. You can go back to the develop module and activate the crop tool. There is a checkbox there to choose to constrain to image. That will eliminate any white space.
Here's what I see, based on Jim's reply:
There is no 'activate the crop tool' per se; It's under Develop module, Lens Correction in the right sidebar; there, you click 'Constrain Crop' and this will crop the image like it was before.
FWIW if you click 'Constrain Crop' again it doesn't 'undo' the crop, but it does remove the checkmark from the box and put another step in your image History.
You need to be working in the develop module. If you do, and activate the crop tool, you have the option to constrain to image. The develop module is where you have the most control over what you are doing.
Wow, OK, yeah even that Basic slider has a lot of options and the Tone Curve is really useful for many of my shots. Thank you.