update-tagblock fails on malformed group field in tagblock

[pwoods25443]

The bug reported in Version: v0.1.5.dev4 (dev branch)

The group tag decode raises ValueError when the group tag is malformed. This example came up in some historical AIS data we were ingesting

\g:1-2--001,c:1326055296*3C\!AIVDM,2,1,3,A,E7`B1:dW7oHth@@@@@@@@@@@@@@6@
6R;mMQM@10888Qr8`8888888888,0*65
\g:2-2--001*41\!AIVDM,2,2,3,A,8880,0*1D

when you run

cat nmea.txt | ais-tools update-tagblock -s 99

you get

Traceback (most recent call last):
  File "/home/paul/.local/lib/python3.9/site-packages/ais_tools/tagblock.py", line 141, in decode_tagblock
    [int(part) for part in value.split("-")])))
  File "/home/paul/.local/lib/python3.9/site-packages/ais_tools/tagblock.py", line 141, in <listcomp>
    [int(part) for part in value.split("-")])))
ValueError: invalid literal for int() with base 10: ''

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/paul/.local/bin/ais-tools", line 8, in <module>
    sys.exit(cli())
  File "/home/paul/.local/lib/python3.9/site-packages/click/core.py", line 1130, in __call__
    return self.main(*args, **kwargs)
  File "/home/paul/.local/lib/python3.9/site-packages/click/core.py", line 1055, in main
    rv = self.invoke(ctx)
  File "/home/paul/.local/lib/python3.9/site-packages/click/core.py", line 1657, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/home/paul/.local/lib/python3.9/site-packages/click/core.py", line 1404, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/home/paul/.local/lib/python3.9/site-packages/click/core.py", line 760, in invoke
    return __callback(*args, **kwargs)
  File "/home/paul/.local/lib/python3.9/site-packages/ais_tools/cli.py", line 104, in update_tagblock
    output.write(tagblock.update_tagblock(nmea.strip(), **fields))
  File "/home/paul/.local/lib/python3.9/site-packages/ais_tools/tagblock.py", line 159, in update_tagblock
    tagblock = decode_tagblock(tagblock_str)
  File "/home/paul/.local/lib/python3.9/site-packages/ais_tools/tagblock.py", line 152, in decode_tagblock
    raise DecodeError('Unable to decode tagblock string')
_ais.DecodeError: Unable to decode tagblock string