ConceptsShared: add a default implementation of BlockMode CryptographicOperation#getBlockMode() for compatibility with external code

python/ql/lib/semmle/python/internal/ConceptsShared.qll

@@ -78,7 +78,7 @@ module Cryptography {
        * This may have no result - for example if the `CryptographicAlgorithm` used
        * is a stream cipher rather than a block cipher.
        */
-      abstract BlockMode getBlockMode();
+      BlockMode getBlockMode() { none() }
     }
   }