Try to be smarter about the "size" column for Catapult export

rattail_corepos/corepos/importing/db/exporters/catapult_inventory.py

@@ -26,6 +26,7 @@ CORE-POS -> Catapult Inventory Workbook
 
 import re
 import datetime
+import decimal
 import logging
 
 from sqlalchemy.exc import ProgrammingError
@@ -407,13 +408,25 @@ class InventoryItemImporter(FromCore, catapult_importing.model.InventoryItemImpo
             if self.exclude_missing_department:
                 return
 
-        size = "{} {}".format((product.size or '').strip(),
-                              (product.unit_of_measure or '').strip())
-        size = size.strip()
+        # size may come from one of two fields, or combination thereof
+        pack_size = (product.size or '').strip()
+        uom = (product.unit_of_measure or '').strip()
+        numeric_pack = False
+        if pack_size:
+            try:
+                decimal.Decimal(pack_size)
+            except decimal.InvalidOperation:
+                pass
+            else:
+                numeric_pack = True
+        if numeric_pack:
+            size = "{} {}".format(pack_size, uom).strip()
+        else:
+            size = pack_size or uom or None
         # TODO: this logic may actually be client-specific?  i just happened to
-        # find some of these chars in a client DB and needed to avoid them, b/c
-        # the openpyxl lib said IllegalCharacterError
-        if '\x00' in size:
+        # find some null chars in a client DB and needed to avoid them, b/c the
+        # openpyxl lib said IllegalCharacterError
+        if size is not None and '\x00' in size:
             logger = log.warning if self.warn_size_null_byte else log.debug
             logger("product %s has null byte in size field: %s",
                    product.upc, product)