[BUG] Incorrect handling of an empty string by the safe access operator

[mexanoz]

Checklist before submitting an issue

• I have searched through the existing closed and open issues for eww and made sure this is not a duplicate
• I have specifically verified that this bug is not a common user error
• I am providing as much relevant information as I am able to in this bug report (Minimal config to reproduce the issue for example, if applicable)

Description of the bug

The safe access operator (?.) fails to recognize an empty string on the left side and return null (as it should according to the documentation), instead throwing an error.

Reproducing the issue

The way I encountered this issue was using a deflisten on playerctl output, which outputs a JSON string but produces an empty string when all players are shut down. Here's a minimal eww.yuck (for Wayland):

(deflisten playerctlstatus 'playerctl metadata -aFf \'{"status": "{{status}}"}\'')

(defwidget playerctl []
    (label :text {playerctlstatus?.status}))

(defwindow test
    :geometry (geometry
                  :width "100%"
                  :height "22px"
                  :anchor "bottom center")
    :exclusive true
    :stacking "fg"
    :monitor 0
    (playerctl))

When there is an active player, the label correctly contains its state. However, when the player shuts down, playerctl outputs "" and the logs show an error:

error: Failed to turn `` into a value of type json-value
  ┌─ /home/mex/.config/eww/test-bug/eww.yuck:4:19
  │
4 │     (label :text {playerctlstatus?.status}))
  │                   ───────────────

Expected behaviour

Either

1. The operator returning a null with an empty string on the left, or
2. The documentation explicitly stating that the operator only works with a null on the left side and not an empty string.

Additional context

No response

[w-lfchen]

will be closed by #1176

note that the docs don't lie, they are just misleading.
currently, the empty string is not supported, a string consisting of 2 " (also referred to as an empty json string) is.

this behavior will be removed by #1176, that pr currently makes indexing "" an error, and adds support for the empty string.