Fix 0 being treated as an invalid subject ID (#122)

`_parse_subject_id()` returns `None` if the string is invalid but the
conditional was only checking for truthiness which 0, a valid subject
ID, does not possess.

Author: optlink

yakut/VERSION

@@ -1 +1 @@
-0.14.0
+0.14.1

yakut/subject_specifier_processor.py

@@ -41,7 +41,7 @@ async def process_subject_specifier(
         sp_sbj_id, sp_dty = specifier.split(":")
         _logger.info("Subject specifier interpreted as explicit: %r", (sp_sbj_id, sp_dty))
         subject_id = _parse_subject_id(sp_sbj_id)
-        if not subject_id:
+        if subject_id is None:
             raise BadSpecifierError(f"{subject_id} is not a valid subject-ID")
         return subject_id, dtype_loader.load_dtype(sp_dty)