WS-523: Make `productfetcher` not explode on null subdirectory value

• Fixed failing on null subdirectory value
• Added a default value validator that will fill in subdirectory if it's None
• test
• Ran successful download of VLBA product

apps/cli/executables/pexable/productfetcher/productfetcher/locations.py

 """ LocationsReport conveniences """
 import http
-from abc import abstractmethod, ABC
+import logging
+from abc import ABC, abstractmethod
 from enum import Enum
 from pathlib import Path
 from typing import NamedTuple, Optional
 
-
 # pylint: disable=E0239, E0401, E0402, E1136, R0201, R0902, R0903, R0913, W0613
 import requests
 from marshmallow import Schema, fields, post_load
+from marshmallow.decorators import validates_schema
 from pycapo import CapoConfig
 
-from .exceptions import LocatorServiceException, FetchError
+from .exceptions import FetchError, LocatorServiceException
 from .interfaces import LocatedFile, LocationReport, Locator
 from .ngas import NgasConnection
 
+logger = logging.getLogger("productfetcher.locations")
+logger.setLevel(logging.INFO)
+
 
 class Location(Enum):
     """
@@ -248,7 +252,7 @@ class NgasFileSchema(Schema):
     """ One of the items in a location report's "files" list """
 
     ngas_file_id = fields.Str()
-    subdirectory = fields.Str()
+    subdirectory = fields.Str(allow_none=True)
     relative_path = fields.Str()
     checksum = fields.Integer()
     checksum_type = fields.Str()
@@ -258,6 +262,12 @@ class NgasFileSchema(Schema):
     table = fields.Str()
     archive_uid = fields.Str()
 
+    @validates_schema
+    def validate_subdirectory(self, data, **kwargs):
+        # Fill in a subdirectory value that is None with the default
+        if data["subdirectory"] is None:
+            data["subdirectory"] = "retrieved-products"
+
     @post_load
     def make_filespec(self, data, **kwargs):
         if "table" in data: