DSC: sparse frame with a non-numeric sender MMSI slips through to a malformed context

[clarkbw]

Follow-up from #336 (review thread r3445099496).

#336 added if (isEmpty(parts[1])) return null before the MMSI substring, which drops empty-MMSI frames ($CDDSC,12,,,07,,,,,,*57) and the bare-$CDDSC,12 TypeError. But a sparse frame with a non-empty but non-numeric MMSI still gets through:

$CDDSC,12,ABC,...

isEmpty is false, so parts[1]!.substring(0, 9) runs on ABC and produces a malformed vessels.urn:mrn:imo:mmsi:ABC context.

Fix is to tighten the parts[1] guard from empty-check to a numeric check (MMSI is always digits per ITU-R M.493). Left out of #336 to keep that PR scoped to distress-alert recognition.

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