parallell-letter-frequency tests are missing some assertions

[ahans]

The tests for the cases that expect an empty map as result are missing the assertions.

The script that I used to generate that code would iterate over the dict and was missing the "empty dict" special case. :(

[github-actions]

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[ahans]

Fix is here: #830 (I only opened this because I expected that issues would not be automatically closed ...)