Pad missing sources as "Unknown"/nan (CASAguide twhya_calibrated.ms failures)

[FedeMPouzols]

Fixes #266, when the field table refers to source IDs missing from the source table, those are filled in the source_xds.

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This contribution is made under the current ALMA software agreements. 
(c) European Southern Observatory, 2024 
Copyright by ESO (in the framework of the ALMA collaboration)

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