Zero-width textprops do not respect start-/end_incl

[axelf4]

For text properties of zero width with start_incl and end_incl set to TRUE, insertions at the position of the textprop do not increase the size of the textprop, which is contrary to what one would expect reading the documentation on start-/end_incl:

"start_incl"    when TRUE inserts at the start position will be
                included in the text property
"end_incl"      when TRUE inserts at the end position will be
                in the text property

To reproduce:

1. Run vim --clean
2. Create text property of length zero with both start_incl and end_incl set to TRUE
3. Insert text at the start position of the created textprop
4. The length of the textprop stays constant?

Would expect the length to increase with each inserted character, like what happens in the case that the length of the textprop is not zero.

Environment:

• Vim version: 8.2.251
• OS: Ubuntu 18.04
• Terminal: tmux in Alacritty