Improved code coverage

[albi3ro]

This PR handles several lines that were missing code coverage by either:

• explicitly specifying the lines will not be covered by our tests
• fixing logic that won't ever be covered
• adding tests

[codecov]

Codecov Report

Merging #2537 (2d4538c) into master (705bfd5) will increase coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2537      +/-   ##
==========================================
+ Coverage   99.47%   99.53%   +0.05%     
==========================================
  Files         243      243              
  Lines       19413    19405       -8     
==========================================
+ Hits        19312    19314       +2     
+ Misses        101       91      -10     

Impacted Files	Coverage Δ
pennylane/drawer/mpldrawer.py	100.00% <ø> (+1.33%)	⬆️
pennylane/fourier/visualize.py	100.00% <ø> (+1.29%)	⬆️
pennylane/ops/qubit/hamiltonian.py	100.00% <ø> (+0.94%)	⬆️
pennylane/ops/qubit/matrix_ops.py	100.00% <0.00%> (+0.87%)	⬆️
pennylane/ops/functions/generator.py	100.00% <0.00%> (+1.42%)	⬆️
pennylane/fourier/coefficients.py	100.00% <0.00%> (+3.22%)	⬆️

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[antalszava]

Looks good! 🎉 Thank you 🙂