kl: short-circuit for empty ccaches

Such as the ones `ksu` sometimes creates.

$ xxd /tmp/krb5cc_1000.1
0000000: 0504 000c 0001 0008 0000 0001 0000 0000  ................
0000010: 0000 0001 0000 0001 0000 0010 4e55 4c4c  ............NULL
0000020: 524f 5554 452e 4555 2e4f 5247 0000 0007  ROUTE.EU.ORG....
0000030: 6772 6177 6974 79                        grawity
kc: symlink to bin/ for easier editing
kc: short-circuit for 'valid' ccaches without any tickets