[xs] fix ZEROPP convergence test (#5061)
there may be chances where the dataset shard loaded contains example
with empty text `''` which will make the test fail (which occurred on my
end) so fixing by dropping the empty examples
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Co-authored-by: Michael Wyatt <michaelwyatt@microsoft.com>
Co-authored-by: Logan Adams <114770087+loadams@users.noreply.github.com>
