humility ibc black-box should skip empty events

[citrus-it]

Fixes #433

jeeves% ./humility ibc black-box
FAULT EVENT
  EVENT_INDEX:        0
  EMPTY
LIFECYCLE EVENT
  EVENT_INDEX:        24
  TIMESTAMP           0xffd5872f = 3 day, 5 hr, 19 min, 4.0 sec
  EVENT_ID            0x0350
  STATUS_WORD         0x0001
    0x0001: System event
  STATUS_MFR          0x05
    0x05: ERASE_OVFL_EVENT
  V_IN                0xf86c = 54.000V
  V_OUT               0x0000 = 0.000V
  I_OUT               0x07ff = -1.000A
  TEMPERATURE         0x0013 = 19.000°C

[bcantrill]

My apologies for the delay on this review! This looks good -- I experimented with different ways to factor it (in particular, to avoid mutable state being passed as a parameter), but I'm not convinced that the result is much cleaner. Pulling this in -- thank you!