fix: output mmsi as string

[sbender9]

fixes #391

[sbender9]

@tkurki this change is causing a test to fail in n2k-signalk

       no User Id produces no output:

      AssertionError: expected 'object' to equal 'undefined'
      + expected - actual

      -object
      +undefined
      
      at Context.<anonymous> (test/129038_ais_class_a.js:87:22)
      at process.processImmediate (node:internal/timers:485:21)

I am inclined to fix the n2k-signalk since canboatjs and canboat output "000000000" in case. Agreed?

[tkurki]

Sure!

[sbender9]

After looking at n2k-signalk, I am now thinking of having the User ID be undefined in this case. That way we won't have backwards compatibility issues.

[tkurki]

Or null? Either may break code that expects a string.

[tkurki]

I mean number.

[sbender9]

Yeah, looks like I would have to return 0 in this case to make the existing n2k-signalk code work. Looks like everything else is ok because n2k-signalk currently calls .toString().