entities: Fix "empty" entity in LaTeX

* lisp/org-entities.el (org-entities): Use \emptyset instead of
\empty, which is a Go symbol.

Arguably, \O outside of math mode would be a better choice, but use
\emptyset for consistency with "emptyset" entity.

lisp/org-entities.el

@@ -386,7 +386,7 @@ packages to be loaded, add these packages to `org-latex-packages-alist'."
      ("exists" "\\exists" t "∃" "[there exists]" "[there exists]" "∃")
      ("nexist" "\\nexists" t "∃" "[there does not exists]" "[there does not  exists]" "∄")
      ("nexists" "\\nexists" t "∃" "[there does not exists]" "[there does not  exists]" "∄")
-     ("empty" "\\empty" t "∅" "[empty set]" "[empty set]" "∅")
+     ("empty" "\\emptyset" t "∅" "[empty set]" "[empty set]" "∅")
      ("emptyset" "\\emptyset" t "∅" "[empty set]" "[empty set]" "∅")
      ("isin" "\\in" t "∈" "[element of]" "[element of]" "∈")
      ("in" "\\in" t "∈" "[element of]" "[element of]" "∈")