Fix audio normalizing for mute clips

[jcbrockschmidt]

Resolves issue #1388. Code that only runs when the fix is applied can be found in tests/test_fx.py within test_normalize_muted, and here for convenience:

make_frame = lambda t: np.array([0.0])
clip = AudioClip(make_frame, duration=1, fps=44100)
clip = audio_normalize(clip)

• I have provided code that clearly demonstrates the bug and that only works correctly when applying this fix
• I have added suitable tests demonstrating a fixed bug or new/changed feature to the test suite in tests/
• I have properly documented new or changed features in the documentation or in the docstrings
• I have properly explained unusual or unexpected code in the comments around it
• I have formatted my code using black -t py36

[jcbrockschmidt]

Does this change warrant a change to the docstring for audio_normalize (check box 3)?

Also, does this change warrant a comment explaining what is happening (check box 4), or is it simple enough to be left as is?

[coveralls]

Coverage increased (+0.02%) to 67.546% when pulling abde0d7 on jcbrockschmidt:fix-normalize-muted into 6ab3efb on Zulko:master.

[tburrows13]

Hey, sorry for the late response. This looks great, and I like your docstring change and comment.
I'll merge it once I've added in a changelog entry.

[mondeja]

Thank you for the fix. I have some suggestions that could improve your pull request. I'm not sure if @tburrows13 agrees with them, could confirm it?

[mondeja]

For me is perfect. Feel free to merge it @tburrows13
Thank you for this fix @jcbrockschmidt!