fastboot: don't print anything in Status() if the input is empty

Status() is called with an empty string to handle `fastboot oem`
commands.  This currently emits a set of spaces and sets
last_start_time such that the epilog can track the time spent in this
command.  Emitting the spaces is problematic however, since it results
in the follow:

 $ fastboot oem device-info
                                                  (bootloader) Verity mode: false
(bootloader) Device unlocked: true
(bootloader) Device critical unlocked: true
(bootloader) Charger screen enabled: true
OKAY [  0.000s]
Finished. Total time: 0.000s

If we skip emitting the spaces, then we get the correct result:

 $ fastboot oem device-info
(bootloader) Verity mode: false
(bootloader) Device unlocked: true
(bootloader) Device critical unlocked: true
(bootloader) Charger screen enabled: true
OKAY [  0.001s]
Finished. Total time: 0.001s

There are no other uses of Status() with an empty string, so this
changes won't impact other commands.

Bug: 158310284
Test: fastboot formats this and other commands correctly.
Change-Id: I6294acefc65a8399160c0944b3fbc2f2ace919ed