Allow passing undefined to ol.layer.Vector#setStyle

[elemoine]

This PR changes the ol.layer.Vector#setStyle to make it more consistent with the way the ol.layer.Vector constructor treats the style option.

Currently, if the style option is undefined or null, the ol.layer.Vector constructor uses the default style function as the style. The setStyle fails (js error) if it is passed null or undefined.

This PR makes the constructor and the setStyle behave in a similar way. If the style passed to the function is null then a null style (and an undefined style function) is used. If undefined is passed then the default style function is used.

This allows to easily reset a vector layer to using the default style, without having to export the ol.style.defaultStyleFunction symbol.

Please review.

[probins]

I added a link to the constructor option docs so people can easily find out what the default style is. It might be a good idea to add the same text to setStyle docs.

[elemoine]

Yep, thanks, I can add that.

[fredj]

(setStyle instead of setCenter in the PR description ?)

[elemoine]

(setStyle instead of setCenter in the PR description ?)

Fixed. Thanks.

[tonio]

Makes sense. Code looks good.

[fredj]

LGTM

[elemoine]

I've been asked why the default style is not used when null is passed. In other words why are null and undefined treated differently?

That use-case: I want that the styles set in the features be used, and I want that features with no styles be not rendered in the layer. This use-case is covered by doing layer.setStyle(null). This is explained in the setStyle API docs.

[ahocevar]

I think this makes sense.