Apply lineJoin style at the first point in radar charts

[nagix]

lineJoin style is not applied to the first point in radar charts. This PR fixes it by adding ctx.closePath if the line has a close path. If it is open, it shifts elements in the point array so that the line is correctly joined at the first point.

I will add image tests in #6263 if this PR is merged first.

Master: https://jsfiddle.net/nagix/bf2geq3o/

This PR: https://jsfiddle.net/nagix/e36dp7k8/

[kurkle]

Just a random note: With data: [1, 1, null, 1, 1], miter gets cropped by chart area.

[kurkle]

IMO that mock canvas test should be removed.

[nagix]

@kurkle The radar chart create a gap with NaN but not with null. This is not consistent with the line chart, so we may need to fix if it's a bug.

[nagix]

I agree to remove mock tests if we add image tests.

[nagix]

Null value resetting to minimum was introduced in #1436, but I'm not sure what this is for. @etimberg Do you have any comment on this?

[nagix]

@kurkle I may have misunderstood your comment - I think the cropped line border with miter is not an issue because it could happen in other chart types and it can be controlled using layout.padding.

On the other hand, null handing is still in question.

[kurkle]

null handling can be done separately if needed.

[etimberg]

@nagix not sure why the null handling was added there. I think it should be skipped like other charts

[nagix]

Removed loop line mock tests.

[nagix]

#6282 was created to fix the null handling.