This bug was introduced in the fix of MDEV-12123, which invoked
page_zip_write_header() in the wrong way.

page_zip_write_header(): Assert that the length is not zero, to
be compatible with page_zip_parse_write_header().

btr_root_raise_and_insert(): Update the uncompressed page and then
invoke page_zip_write_header() with the correct length.