0-length path, square cap not rendered (unlike in Firefox)

Uncertain if the error is theirs or ours, the bounding box in Inkscape indicates its ours.

Steps to reproduce:

• open this custom file and select all
• compare to firefox or browser rendering

firefox rendering	svg	selected in Inskcape	png export

What happened?

For the following paths (with cap: square), no rendering is visible (unlike in firefox)
M 5,5 v 0, M 5,5 v 0 0, M 5,5 l 0,0 0,0, m 5,5 0,0 0,0

Note that when the path has two nodes, the selection is a square, but for 3+ nodes, it has the bug described in #1674 (moved)

Inkscape Version and Operating System:

Inkscape 1.1-dev (4fa9aefd53, 2020-01-24) debug build, Linux Mint 19.1
Inkscape 0.92.3 ppa Linux Mint 19.1

Edit: File updated (previously changed by optimizations)