[Fixbug 18418] [Yang] MACSEC_PROFILE table does not match yang definition (#18419)

### Why I did it
Fix the issue [18418](#18418), The YANG model to MACsec fallback cak/ckn is wrong. We should allow the fallback option is empty.

### How I did it
Add a condition to check whether the fallback is providered

#### How to verify it
Check Azp

Author: Pterosaur

src/sonic-yang-models/tests/yang_model_tests/tests_config/macsec.json

@@ -30,6 +30,18 @@
                         "replay_window": 64,
                         "send_sci": "true",
                         "rekey_period": 3600
+                    },
+                    {
+                        "name": "test_nofallback",
+                        "priority": 64,
+                        "cipher_suite": "GCM-AES-XPN-256",
+                        "primary_cak": "5207554155500e5d5157786d6c2a3d2031425a5e577e7e727f6b6c03312432262706080a00005b554f4e007975707670725b0a54540c0252445e5d7a29252b046a",
+                        "primary_ckn": "6162636465666768696A6B6C6D6E6F706162636465666768696A6B6C6D6E6F70",
+                        "policy": "security",
+                        "enable_replay_protect": "true",
+                        "replay_window": 64,
+                        "send_sci": "true",
+                        "rekey_period": 3600
                     }
                 ]
             }

src/sonic-yang-models/yang-models/sonic-macsec.yang

@@ -70,9 +70,9 @@ module sonic-macsec {
                     }
                 }
 
-                must "string-length(fallback_cak) = string-length(primary_cak)";
+                must "string-length(fallback_cak) = 0 or string-length(fallback_cak) = string-length(primary_cak)";
 
-                must "primary_ckn != fallback_ckn";
+                must "string-length(fallback_ckn) = 0 or primary_ckn != fallback_ckn";
 
                 leaf policy {
                     type string {