relax bds17 identification

pyModeS/decoder/bds/__init__.py

@@ -101,7 +101,7 @@ def infer(msg):
     df = common.df(msg)
 
     if common.allzeros(msg):
-        return None
+        return 'EMPTY'
 
     # For ADS-B / Mode-S extended squitter
     if df == 17:

pyModeS/decoder/bds/bds17.py

@@ -46,7 +46,11 @@ def is17(msg):
 
     # basic BDS codes for ADS-B shall be supported
     #   assuming ADS-B out is installed (2017EU/2020US mandate)
-    if not set(['BDS05', 'BDS06', 'BDS08', 'BDS09', 'BDS20']).issubset(caps):
+    # if not set(['BDS05', 'BDS06', 'BDS08', 'BDS09', 'BDS20']).issubset(caps):
+    #     return False
+
+    # at least you can respond who you are
+    if 'BDS20' not in caps:
         return False
 
     return True