fix: vi-mode chevron enabled when vi-mode disabled

[RWejlgaard]

Description

On some distributions the ending chevron would be pointing to the left when it should point to the right.

Motivation and Context

Tide figured out "is the user on emacs or vi bindings?" by comparing $fish_key_bindings to the literal string fish_default_key_bindings.

Problem: most emacs users never set that variable at all. Fish just uses emacs bindings by default and leaves the variable empty. So the equality check came back false and we fell through to the vi switch. Since $fish_bind_mode is default whether you're in vi command mode or just using plain emacs bindings, the prompt happily rendered ❮ (and the vi_mode item lit up) for people who had nothing to do with vi mode.

This PR treats an empty $fish_key_bindings the same as the explicit default, so vi rendering only kicks in when someone actually opted into vi bindings.

Closes #640 #641 #622 #625

Screenshots (if appropriate)

How Has This Been Tested

Tested on my own machine (MacOS) and on linux via docker containers.

• I have tested using Linux.
• I have tested using MacOS.

Checklist

• I am ready to update the wiki accordingly.
• I have updated the tests accordingly.