row-count: require f:c, not file:size

I tried using the empty List() and got a test failure, so it seems like
we do need to specific *some* field here.

file:size gets populated by the extraction job, not the backfill job, so
I had been miscounting table sizes (counting only the number of GROBID
extracted items, not rows in the table).

TODO: count on key or no column, not f:c

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