Inconsistent RDB$COLLATION_ID when restoring from 2.5 to 3.0 for same UDF [CORE6081]

[firebird-automations]

Submitted by: @cincuranet

Steps:
1. Create empty database on 2.5.
2. Create this UDF:
DECLARE EXTERNAL FUNCTION BIGTIMESPAN_TO_STRING
BIGINT
RETURNS CSTRING(20) FREE_IT
ENTRY_POINT 'BigTimeSpanToString' MODULE_NAME 'Fast';
3. Backup the database using gbak.
4. Restore the database on 3.0 using gbak.
5. Check RDB$FUNCTION_ARGUMENTS.RDB$COLLATION_ID for the return argument. It's NULL.
5. Create the same UDF again (either new name or drop+create).
6. Check RDB$FUNCTION_ARGUMENTS.RDB$COLLATION_ID for the return argument again. It's non-NULL.

Same function, same declaration, different result.