Default prompt / __fish_print_pipestatus can't handle TERM=dumb

[zanchey]

fish, version 3.2.1-90-ge56d8a8e5

env -i HOME=(mktemp -d) TERM=dumb ~/prefix/bin/fish
Welcome to fish, the friendly interactive shell          
Type help for instructions on how to use fish                                                                                  
error: missing argument                      
in function '__fish_print_pipestatus' with arguments '\[ \] \| 0'
        called on line 1 of file /home/wheel/zanchey/prefix/share/fish/functions/fish_prompt.fish
in command substitution      
        called on line 33 of file /home/wheel/zanchey/prefix/share/fish/functions/fish_prompt.fish
in function 'fish_prompt'        
in command substitution

This is because __fish_print_pipestatus gets called with a number of set_color command substitutions as arguments, which are empty when TERM is "dumb".

[faho]

This is a specific manifestation of #5443 - if set_color printed something sensible when run with no arguments (or TERM=dumb) this wouldn't be a problem.

I still have no idea what, tho - some no-op escape sequence would be great. The alternative is to have some way to quote a command substitution - "$(set_color)" would just expand to the same as "", an empty string.

Still worth fixing here, tho, and ideally in __fish_print_pipestatus. We might just want to use argparse and pass the colors as options, this current argument passing scheme is awkward.