Pareto k tail fix

[n-kall]

Summary

As brought up by @avehtari in issue #345, when one tail is constant, but tail = "both" is specified, it is better to return the diagnostic for the non-constant tail rather than NA.

Copyright and Licensing

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.29%. Comparing base (5403ae5) to head (8435f4a).

❗ Current head 8435f4a differs from pull request most recent head 23d3e9a. Consider uploading reports for the commit 23d3e9a to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #351      +/-   ##
==========================================
- Coverage   95.31%   95.29%   -0.02%     
==========================================
  Files          50       50              
  Lines        3840     3845       +5     
==========================================
+ Hits         3660     3664       +4     
- Misses        180      181       +1     

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[paul-buerkner]

Looks sensible to me. Any more changes you want to do before I merge?

[github-actions]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if 8435f4a is merged into master:

•   :ballot_box_with_check:as_draws_array: 144ms -> 144ms [-1%, +0.95%]
• ❗🐌as_draws_df: 74.5ms -> 75.6ms [+0.77%, +2.09%]
•   :ballot_box_with_check:as_draws_list: 178ms -> 178ms [-1.84%, +1.1%]
• ❗🐌as_draws_matrix: 62.4ms -> 63.7ms [+0.55%, +3.42%]
•   :ballot_box_with_check:as_draws_rvars: 85.3ms -> 86.1ms [-0.27%, +2.18%]
•   :ballot_box_with_check:summarise_draws_100_variables: 709ms -> 707ms [-0.63%, +0.1%]
•   :ballot_box_with_check:summarise_draws_10_variables: 111ms -> 111ms [-0.82%, +1.43%]
  Further explanation regarding interpretation and methodology can be found in the documentation.

[n-kall]

I don't have more to add regarding this, you can merge