Fix empty strings in AnymailInboundMessage from/to/cc

Fix AnymailInboundMessage.to, .cc, .from_email when
message was built with AnymailInboundMessage.construct
using empty strings for those params. (Postmark inbound
relies on this.)

Fixes #307

anymail/inbound.py

@@ -113,7 +113,10 @@ class AnymailInboundMessage(Message):
         """
         values = self.get_all(header)
         if values is not None:
-            values = parse_address_list(values)
+            if "".join(values).strip() == "":
+                values = None
+            else:
+                values = parse_address_list(values)
         return values or []
 
     def get_date_header(self, header):