bash: always send "end of command" prompt sequence

I noticed that both wezterm[0] and the OSC 133 reference script[1] (upon
which wezterm and our bash integration appear to be based) always send
the "end of prompt" sequence in their precmd functions (except during
their first execution, when no previous command has been executed).

This is a subtle logical difference: `$_ghostty_executing=""` (first
run) versus `$_ghostty_executing="0"` (reentrancy check). I'm not sure
how much a difference it makes except in rare edge case scenarios, but I
think it makes sense to be consistent and always report the end of the
current command.

[0] https://github.com/wez/wezterm/blob/main/assets/shell-integration/wezterm.sh#L479
[1] https://gitlab.freedesktop.org/Per_Bothner/specifications/-/blob/master/proposals/prompts-data/shell-integration.bash

src/shell-integration/bash/ghostty.bash

@@ -140,7 +140,8 @@ function __ghostty_precmd() {
       fi
     fi
 
-    if test "$_ghostty_executing" != "0"; then
+    if test "$_ghostty_executing" != ""; then
+      # End of current command. Report its status.
       builtin printf "\033]133;D;%s;aid=%s\007" "$ret" "$BASHPID"
     fi
 
@@ -152,6 +153,7 @@ function __ghostty_precmd() {
       builtin printf "\e]7;kitty-shell-cwd://%s%s\a" "$HOSTNAME" "$PWD"
     fi
 
+    # Fresh line and start of prompt.
     builtin printf "\033]133;A;aid=%s\007" "$BASHPID"
     _ghostty_executing=0
 }