Series: Test is not testing for zero length series

[kotp]

This test now raises if the series is 0 instead of of the slice slice
size being larger than the series.

This allows one to raise on normalization of user input, the series
given, rather than incidentally having the slice size of 1 be larger
than the series size from a zero length string.

The example solution is updated to raise specifically for a zero length
series, rather than the incidental raising when slices are asked for.

[github-actions]

This PR touches files which potentially affect the outcome of the tests of an exercise. This will cause all students' solutions to affected exercises to be re-tested.

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[no important files changed]

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