None safety

src/relstorage/cache/cache.pyx

@@ -482,18 +482,18 @@ cdef class PyCache:
     def __setitem__(self, OID_t key, tuple value):
         # Do all this down here so we don't give up the GIL.
         cdef TID_t tid = value[1]
-
+        state = value[0] or b''
         if not self.cache.contains(key): # the long way to avoid type conversion
             self.cache.add_to_eden(
                 key,
-                PyBytes_AsString(value[0] or b''),
-                len(value[0]),
+                PyBytes_AsString(state),
+                len(state),
                 tid
             )
         else:
             # We need to merge values.
             try:
-                self.cache.store_and_make_MRU(key, PyBytes_AsString(value[0]), len(value[0]), tid)
+                self.cache.store_and_make_MRU(key, PyBytes_AsString(state), len(state), tid)
             except RuntimeError as e:
                 raise CacheConsistencyError(str(e))