fix empty block handling

Avoid BlockDoesNotExistsException on `blockchain.stream()` and `Block()`
with `only_ops=True` on blocks without ops.
Fixes holgern#149.

beem/block.py

@@ -141,7 +141,9 @@ def refresh(self):
                          'timestamp': ops[0]["timestamp"],
                          'operations': ops}
             else:
-                block = {}
+                block = {'block': self.identifier,
+                         'timestamp': "1970-01-01T00:00:00",
+                         'operations': []}
         else:
             if self.steem.rpc.get_use_appbase():
                 try: