use explicit check instead of implicit checking the part for truthy as to…

[jhonalino]

… not skip part 0

This PR will...

fix the part check conditional for the logic that overrides the level/track timeouts for LL-HLS blocking playlist reload request

Why is this Pull Request needed?

the level/track timeouts override is required for all parts, but its skipping part 0 erroneously because 0 is falsy,

Are there any points in the code the reviewer needs to double check?

skipping part 0 maybe on purpose?

Resolves issues:

not overriding the level/track timeouts for part 0 for LL-HLS

Checklist

• changes have been done against master branch, and PR does not conflict
• new unit / functional tests have been added (whenever applicable)
• API or design changes are documented in API.md

[robwalch]

Thanks @jhonalino!

I'll merge this shortly after the build checks are complete.

Note: the player is overriding config.playlistLoadPolicy.default which is expected to be tuned for VOD and standard Live playlist loading. If you think it would be a useful feature for hls.js to define a policy specifically for Low-Latency playlist loading that developers could override, please consider filing a feature request. Adding config.playlistLoadPolicy.lowLatency might be a good addition, and would allow this code to be removed. Static defaults might not work as well as these dynamic settings for every part target duration, but 1 second is the norm, and it would be fully configurable.