rxrpc: Fix trace string

The trace symbol printer (__print_symbolic()) ignores symbols that map to
an empty string and prints the hex value instead.

Fix the symbol for rxrpc_cong_no_change to " -" instead of "" to avoid
this.

Fixes: b54a134a

 ("rxrpc: Fix handling of enums-to-string translation in tracing")
Signed-off-by: David Howells <dhowells@redhat.com>