Fix panic if feedback_array.size == 0

[kumattau]

@wez

This PR fixes the following panic if feedback_array.size == 0.
I encountered at running wezterm on X11 in WSLg.

related to: wez/wezterm#2056

unsafe precondition(s) violated: slice::from_raw_parts requires the pointer to be aligned and non-null, and the total size of the slice not to exceed `isize::MAX`

[wez]

Ah, thanks! I pushed a slightly different version of this that applies to each from_raw_parts call, and updated wezterm main to reference the result.

[kumattau]

@wez

Thank you for update.

358e226 returns empty array if ptr.is_null(), but I think it is better to return empty array if size == 0.

It is because ptr.is_null() && size != 0 means that something wrong occurs outside of xcb-imdkit-rs, so this case should panic and on the other side, !ptr.is_null() && size == 0 can be interpretted as empty array.

Of cause, I'm okay to keep the current ptr.is_null() check because it works without problem in usual case.