refs codership/mysql-wsrep#239

Synced xtrabackup SST scripts from PXC source tree as of PXC 5.6.27-25.13
- PXC#480: xtrabackup-v2 SST fails with multiple log_bin directives in my.cn
- PXC#460: wsrep_sst_auth don't work in Percona-XtraDB-Cluster-56-5.6.25-25.
- PXC-416: Fix SST related issues.
- PXC-389: Merge remote-tracking branch 'wsrep/5.6' into 5.6-wsrep-pxc389
- Bug #1431101: SST does not clobber backup-my.cnf

scripts/wsrep_sst_xtrabackup.sh

@@ -436,15 +436,17 @@ then
 
     if [ $WSREP_SST_OPT_BYPASS -eq 0 ]
     then
+        usrst=0
         TMPDIR="${TMPDIR:-/tmp}"
 
-        if [ "$WSREP_SST_OPT_USER" != "(null)" ]; then
+        if [[ -n "${WSREP_SST_OPT_USER:-}" && "$WSREP_SST_OPT_USER" != "(null)" ]]; then
            INNOEXTRA+=" --user=$WSREP_SST_OPT_USER"
+           usrst=1
         fi
 
         if [ -n "${WSREP_SST_OPT_PSWD:-}" ]; then
            INNOEXTRA+=" --password=$WSREP_SST_OPT_PSWD"
-        else
+        elif [[ $usrst -eq 1 ]];then
            # Empty password, used for testing, debugging etc.
            INNOEXTRA+=" --password="
         fi