MDL-71397 mod_glossary: empty category is the same as all categories

In PHP8 empty string is not equal to zero

mod/glossary/lib.php

@@ -1949,7 +1949,7 @@ function glossary_print_categories_menu($cm, $glossary, $hook, $category) {
             echo get_string("entrieswithoutcategory","glossary");
             $selected = GLOSSARY_SHOW_NOT_CATEGORISED;
 
-        } elseif ( $hook == GLOSSARY_SHOW_ALL_CATEGORIES ) {
+        } else if ( empty($hook) ) {
 
             echo get_string("allcategories","glossary");
             $selected = GLOSSARY_SHOW_ALL_CATEGORIES;