StopIteration on empty input

[jpmckinney]

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To reproduce:

printf '' | python -c 'import sys; import agate; agate.Table.from_csv(sys.stdin)'

Note that examples/empty.csv (in agate and csvkit) contains a single newline, which is sufficient to not error.

If this issue is closed, can add tests for empty inputs in csvkit (only affects tools that buffer).