MySQL/MariaDB backup with DB0X_SPLIT_DB=FALSE produces empty dump (undefined $mysqldump_bin instead of $_mysqldump_bin) #496

Description

When DB0X_SPLIT_DB=FALSE (the default, used for combined "ALL databases" dumps),
the MySQL/MariaDB backup command silently produces an empty output file instead
of a real dump.

Root cause

In rootfs/container/functions/10-dbbackup, the dump command differs between
the two DB0X_SPLIT_DB code paths:

• SPLIT_DB=TRUE (per-database dump, line ~620):
  ${_mysql_prefix}${_mysqldump_bin} ... — correct.
• SPLIT_DB=FALSE (combined "ALL" dump, line ~641):
  ${_mysql_prefix}${mysqldump_bin} ... — missing the leading underscore.

_mysqldump_bin (with underscore) is the variable actually set earlier in the
client-selection block (mariadb-dump or mysqldump, depending on
DB0X_MYSQL_CLIENT). mysqldump_bin (without underscore) is never assigned
anywhere in the image. In the SPLIT_DB=FALSE branch it therefore expands to
an empty string, so the command that actually runs is just
${_mysql_prefix} (e.g. /usr/bin/ or /opt/mysql/bin/) with all the dump
flags as arguments - which fails immediately and produces zero stdout. That
gets piped into the compression step regardless, so the job "succeeds" and
writes a tiny (e.g. 13-byte) file that looks like a valid but empty backup -
no error is surfaced at the top level.

Steps to reproduce

1. Run the container against any MySQL or MariaDB server with:
   • DB01_TYPE=mysql (or mariadb)
   • DB01_NAME=ALL
   • DB01_SPLIT_DB=FALSE (or unset, since FALSE is the default)
   • valid DB01_HOST/DB01_USER/DB01_PASS
2. Let the backup job run (MODE=MANUAL, backup-now, or the scheduler).
3. Inspect the resulting .sql.zst/.sql.gz file in the backup location.

Expected

A real SQL dump of all databases.

Actual

A near-empty file (just the compression format's empty-stream header, e.g.
13 bytes for zstd) - mysqldump/mariadb-dump was never actually invoked.

Suggested fix

In the SPLIT_DB=FALSE branch, change:

${_mysql_prefix}${mysqldump_bin}

to:

${_mysql_prefix}${_mysqldump_bin}

matching the SPLIT_DB=TRUE branch above it.

Environment

• Image: docker.io/nfrastack/db-backup
• Confirmed via source inspection against the current default branch.