MDEV-31423: Make sure that datadir is available with SySV-init script

Commit fixes Debian SySV-init script fail:
  /etc/init.d/mariadb: line 90: [: : integer expression expected
which happens if datadir is not changed in configuration which
makes it invisible when printing MariaDB config defaults.

Commit makes sure that there is some value if nothing else if in hand
use default /usr/lib/mysql or fail with correct error message if
directory is not present

debian/mariadb-server-10.6.mariadb.init

@@ -84,6 +84,24 @@ sanity_checks() {
 
   # check for diskspace shortage
   datadir=`mariadbd_get_param datadir`
+
+  # If datadir location is not changed int configuration
+  # then it's not printed with /usr/sbin/mariadbd --print-defaults
+  # then we use 'sane' default.
+  if [ -z "$datadir"]
+  then
+    datadir="/var/lib/mysql"
+  fi
+
+  # Check if there datadir location is available and
+  # fail if it's not
+  if [ ! -d "$datadir" ]
+  then
+    log_failure_msg "$0: ERROR: Can't locate MariaDB installation location $datadir"
+    echo                "ERROR: Can't locate MariaDB installation location $datadir" | $ERR_LOGGER
+    exit 1
+  fi
+
   # As preset blocksize of GNU df is 1024 then available bytes is $df_available_blocks * 1024
   # 4096 blocks is then lower than 4 MB
   df_available_blocks="$(LC_ALL=C BLOCKSIZE='' df --output=avail "$datadir" | tail -n 1)"