After coalescing the root's two children, the separator is erased leaving
the root with 0 keys and 1 child. adjust_root previously only handled
size==1 (dead branch) and size==0 leaf, so the internal root stayed
corrupt (0 keys, 1 child), causing cascading erase_pair assertions and
server crashes during deletes. Replace the size==1 branch with size==0
internal handling: promote the only child as the new root.

Verified via bpt_test stress (insert/delete/merge/split) and full
unit_test suite.