Description of the bug

Upon upgrading 4 sites from 1.31.1 to 1.32.0, the upgrade goes well.
However, when update.php is run, there are 2 updates pending, but they end up with the following fatal errors:

Deprecated function: unserialize(): Passing null to parameter #1 ($data) of type string is
deprecated in system_update_1103() (line 3814 
of /foo/backdrop/core/modules/system/system.install).

PDOException: SQLSTATE[42S02]: Base table or view not found: 1146 Table 'be.cache_book' 
doesn't exist: TRUNCATE {cache_book} ; Array ( ) in backdrop_flush_all_caches() (line 8910 
of /foo/backdrop/core/includes/common.inc).

If it matters: schema version in system table for book module is 6000 now, after the update (not 7000 as I initially reported).

The site is left in maintenance mode, which has to be turned off manually.

Steps To Reproduce

To reproduce the behavior:

1. Starting with a 1.31.1 site, upgrade to 1.32.0
2. Run update.php
3. Observer the above errors
4. Note that the maintenance mode message will be on

Actual behavior

1. Fatal errors upon running update.php
2. Site left in maintenance mode

Expected behavior

1. There should be no fatal errors
2. The site should not be left in maintenance mode

Additional information

None