udev: skip empty device-mapper nodes

It can happen during device reconfiguration - do not decide to expose
the device until its known what device it will be.
This fixes bug where root.img was visible in qvm-block as normal device
and could be detached.

udev/udev-qubes-block.rules

@@ -12,6 +12,7 @@ ENV{MAJOR}=="202", GOTO="qubes_block_end"
 
 # Skip device-mapper devices
 ENV{MAJOR}=="253", ENV{DM_NAME}=="snapshot-*", GOTO="qubes_block_end"
+ENV{MAJOR}=="253", ENV{DM_NAME}=="", GOTO="qubes_block_end"
 
 IMPORT{db}="QUBES_EXPOSED"
 ACTION=="add", IMPORT{program}="/usr/bin/flock /var/run/qubes-udev.lock -c /usr/libexec/qubes/udev-block-add-change"