Fix error when evaluating buffer directly

[zonuexe]

Handle cases where load-file-name is nil by falling back to default-directory. This allows the code to be evaluated directly without errors.

[manateelazycat]

No, it's not right, if default-directory it not acm/ directory, code can pass, but result is wrong

[zonuexe]

This is a common pattern in Emacs Lisp to support both loading and direct evaluation. Similar approaches are used in other projects as well:

• https://github.com/abo-abo/swiper/blob/ec9421340c88ebe08f05680e22308ed57ed68a3d/ivy.el#L5493-L5501
• https://github.com/manateelazycat/holo-layer/blob/dc7d48338b7c243fe5660be40cedb5af9e0c94c5/holo-layer.el#L88-L90

After submitting this PR, I realized that the following might be a more concise way to handle it:

(defvar acm-icon-dir (expand-file-name "icons" (file-name-directory (or load-file-name buffer-file-name))))

Since this is not a critical issue, I don't intend to pursue this specific change further or update the PR. However, I remain happy to contribute to the project's development in other areas. Thanks!