Small fix for edge case where planned or prepared have now rows in db

[ainefairbrother]

Two small fixes:

• Added in conditional logic for when planned or prepared status metadata db query yields no result - don't apply .pop, set variable to None and don't write JSON file, but do notify that this has occurred in the log file.
• Prevent warning being thrown erroneously when planned_ids.pop() or prepared_ids.pop() evaluates to an empty set with length 0 - this was throwing a WARN that there were multiple unique IDs due to !=1