Use the unit type correctly instead of null references.

include/libabacus.h

@@ -182,6 +182,13 @@ void libab_get_type_function_list(libab* ab, libab_ref* into);
  */
 void libab_get_type_unit(libab* ab, libab_ref* into);
 
+/**
+ * Gets the unit value form this libab instance.
+ * @param ab the instance to get the unit value from.
+ * @param into the reference into which to store the unit value.
+ */
+void libab_get_unit_value(libab* ab, libab_ref* into);
+
 /**
  * Executes the given string of code.
  * @param ab the libabacus instance to use for executing code.