Skip rendering arrow wrapper when custom arrow renderer returns falsy value

[mtlewis]

Closes #1700.

[coveralls]

Coverage increased (+0.02%) to 92.368% when pulling 93af361 on mtlewis:skip-arrow-zone-when-arrow-renderer-returns-falsy into 7523c20 on JedWatson:master.

[lifejuggler]

Wouldn't it be better if there is a separate option to disable the arrow functionality like disableArrow: true/false instead of an override like this?

[mtlewis]

I don’t have strong feelings either way, but my reasoning for doing it this way is that it avoids having two props which have to have an order of precedence. I do agree that a disableArrow prop would make the ‘no arrow’ use case more readable though!

[lifejuggler]

Same for me just that it would be easier to use the default arrowRenderer when someone wants them, but I am fine with this change... just wish it would get merged ASAP

[mtlewis]

Just a note that we've worked around this for our use case by adding a rule like the below, which is not as neat as having react-select support skipping arrow rendering completely, but does get the functionality to where we want it today.

.myReactSelect .Select-arrow-zone {
  display: none;
}

[JedWatson]

I think this makes sense as a fix for the problem, and is better than adding another prop.

Thanks @mtlewis!