Prevent nan in the rms calculation

[mgullik]

Also, it checks the last frequency allowed to compute rms
Add tests

[codecov]

Codecov Report

Merging #736 (e62358a) into main (3a7e816) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #736      +/-   ##
==========================================
- Coverage   97.17%   97.15%   -0.03%     
==========================================
  Files          42       42              
  Lines        7865     7872       +7     
==========================================
+ Hits         7643     7648       +5     
- Misses        222      224       +2     

Impacted Files	Coverage Δ
stingray/fourier.py	99.74% <100.00%> (+<0.01%)	⬆️
stingray/powerspectrum.py	99.45% <100.00%> (+<0.01%)	⬆️

... and 1 file with indirect coverage changes

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[matteobachetti]

Hi @mgullik, thanks for your PR!
The enable_deprecations_as_warnings issue that makes the devdeps test fail is partially solved in commit #cc2a561bbf770fd269a82b9aa3704c303a36dc2c, feel free to cherry-pick.
For the changelog issue, please add a file called 736.bugfix.rst to docs/changes with a sentence describing your fix

[matteobachetti]

LGTM! Thanks @mgullik, I added a couple of small modifications to fix the unrelated test failures, which will be corrected more robustly in #738.