Panorama edges.

Those edges around the merged panorama are not white, by the way. Technically, the non-image area is stored as transparency in the resulting DNG panorama.

Because the surrounding area is actually transparent, you can open the panorama in a photo editor such as Photoshop and slap a solid black fill layer under it. Should take less than 10 seconds total.

The only edge handling options in Lightroom Classic itself are the ones Rob mentioned.

In LrC, when you [Merge to Panorama] you have two adjustments that can 'fill' the blank areas as the panorama is created. But LrC does not have the capability to adjust these areas after the pano is created (Other than the CROP Tool).

If you have already created the Panorama image, then Photoshop would be the best to fill the 'transparent' areas-

In Photoshop (one method)-

1) Hold [Ctrl] as you click directly on the Layer thumbnail image to select the image pixels

2) Press [Shift + Ctrl + i ] to invert the selection

3) Expand the Selection of transparent areas by a few (5?) pixels (Helps eliminate a fine white line!)

4) Go Menu: Edit > Content Aware Fill.  

A 'Black edge' or border is also something easier to apply in Photoshop.