allow an vaild user login with empty passwd
build: ./configure --host=mipsel-buildroot-linux-uclibc --disable-lastlog --disable-utmp --disable-utmpx --disable-wtmp --disable-wtmpx --disable-loginfunc --disable-pututline --disable-pututxline --enable-static --disable-syslog --disable-zlib
make
run: /mnt/sdcard/dropbear -p 2222 -R -B