encrypt-binlog / encrypt-tmp-files silently write plaintext when the key-management plugin returns ENCRYPTION_KEY_NOT_ENCRYPTED (version 0)

`ENCRYPTION_KEY_NOT_ENCRYPTED` (value 0) is handled as "success without encryption" instead of an
error:

• `sql/mf_iocache_encr.cc:266` — when the key-management plugin reports version 0, the encryption
  hooks are left unset and the function returns success.
• `sql/log.cc:4641` — consequently `Start_encryption_log_event` is not written and the log write
  path returns success.
• `sql/mysqld.cc:5821` — the startup check that should reject the configuration accepts this state.

So a server started with `-encrypt-binlog=ON` (and/or `-encrypt-tmp-files=ON`) can end up writing
plaintext when the configured key-management plugin returns version 0, with no error and no
indication in the log that encryption was silently skipped.

1. 1. Expected result

`encrypt-binlog`/`encrypt-tmp-files` must fail closed: if encryption is requested and the provider
reports that it cannot encrypt (`ENCRYPTION_KEY_NOT_ENCRYPTED`), startup (or the affected log write)
must abort with a clear error rather than continue in plaintext.