fix(tests): fix stderr calculation in test/test_distributions::test_entropy_samples to handle zero standard deviation

[Qazalbash]

FAILED test/test_distributions.py::test_entropy_samples[Beta-<lambda>-params8] - assert (Array(False, dtype=bool) or Array(False, dtype=bool))
FAILED test/test_distributions.py::test_entropy_samples[BetaProportion-<lambda>-params11] - assert (Array(False, dtype=bool) or Array(False, dtype=bool))

These tests were failing due to a stderr being zero. I have fixed it by taking a very small number if there is a zero.

[Qazalbash]

Failing test cases are fixed in #2067.

[fehiepsi]

Thanks @Qazalbash! Could we merge the fixes or mark the current failing tests by xfail so that we can address them in the other PR?

[Qazalbash]

Asking out of curiosity, what's the point of marking them when they are fixed in #2067? Just merge them one after another.

[fehiepsi]

We cant merge PR with failing checks.

[fehiepsi]

Thanks for the fix, @Qazalbash!