fix(lib): doom/delete-this-file nil path handling

Fix the handling of a nil path within doom/delete-this-file.

If path is nil (e.g. called interactively when buffer is not visiting a
file), avoid calling abbreviate-file-name on nil, otherwise an error
will be signaled:

  (wrong-type-argument stringp nil)

Additionally, fix the subsequent path checks. These were treating two
distinct scenarios as a "Buffer is not visiting any file" user-error:

- nil path
- non-existent path
- Only the first should result in that error. The second should proceed
  to the next path check (which was previously unreachable), to signal
  the appropriate error, "File doesn't exist: %s".

lisp/lib/files.el

@@ -405,8 +405,8 @@ If FORCE-P, delete without confirmation."
    (list (buffer-file-name (buffer-base-buffer))
          current-prefix-arg))
   (let* ((path (or path (buffer-file-name (buffer-base-buffer))))
-         (short-path (abbreviate-file-name path)))
-    (unless (and path (file-exists-p path))
+         (short-path (and path (abbreviate-file-name path))))
+    (unless path
       (user-error "Buffer is not visiting any file"))
     (unless (file-exists-p path)
       (error "File doesn't exist: %s" path))