Height-less Grid still displays header nodes

[benschell]

After some testing related to #5343, I've found that, even when the Grid sizes itself to 0px tall (due to it's parent not having an appropriate height set), the column headers still appear. If the Grid is sizing itself to a zero height, the column headers should be hidden as well, otherwise the Grid appears broken (when in fact it's just not displayed).

[benschell]

I've included a modification to the test_sizing.html test file which includes a Grid instance in an unsized container. The behavior can be seen with the given fix not included. The rest of the patch corrects the behavior.

[benschell]

Fixed absolute paths in previous patch

[jaredj]

Talked with Ben on this and I agree with his fix. If the parent container has 0 size, then nothing of the grid should display. Otherwise, it does appear broken that the header displays, but no data. It tends to suggest that the data model is wrong somehow when it actually wasn't.

[jaredj]

Minor cleanup of Ben's patch.

[jaredj]

(In [12420]) Applying patch from Ben Shell. Minor fix. Fixes #5761 !strict