Allow NULL Buffer Profile in buffer pg/queue yang files for Dynamic B… (#10353)

Why I did it
To address #10342 (Yang validation fails when Buffer profiles are configured with "NULL" values)

src/sonic-yang-models/tests/yang_model_tests/tests/buffer_pg.json

@@ -10,6 +10,9 @@
         "desc": "BUFFER_PG_WRONG_PG_VALUE pattern failure",
         "eStr": "Invalid Buffer PG number"
     },
+    "BUFFER_PG_NULL_PROFILE_VALUE": {
+        "desc": "BUFFER_PG_NULL_PROFILE_VALUE no failure"
+    },
     "BUFFER_PG_WRONG_PORT_VALUE": {
         "desc": "BUFFER_PG_WRONG_PORT_VALUE pattern failure",
         "eStr": "wrong"

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

@@ -52,6 +52,58 @@
             }
         }
     },
+    "BUFFER_PG_NULL_PROFILE_VALUE": {
+        "sonic-port:sonic-port": {
+            "sonic-port:PORT": {
+                "PORT_LIST": [
+                {
+                    "admin_status": "up",
+                    "alias": "eth0",
+                    "description": "Ethernet0",
+                    "lanes": "65",
+                    "mtu": "9000",
+                    "name": "Ethernet4",
+                    "tpid": "0x8100",
+                    "speed": "25000"
+                }
+                ]
+            }
+        },
+        "sonic-buffer-pool:sonic-buffer-pool": {
+            "sonic-buffer-pool:BUFFER_POOL": {
+                "BUFFER_POOL_LIST": [
+                {
+                    "name": "egress_lossless_pool",
+                    "mode": "static",
+                    "size": "300",
+                    "type": "ingress"
+                }
+                ]
+            }
+        },
+        "sonic-buffer-profile:sonic-buffer-profile": {
+            "sonic-buffer-profile:BUFFER_PROFILE": {
+                "BUFFER_PROFILE_LIST": [
+                {
+                    "name": "lossless_buffer_profile",
+                    "size": "300",
+                    "pool": "egress_lossless_pool"
+                }
+                ]
+            }
+        },
+        "sonic-buffer-pg:sonic-buffer-pg": {
+            "sonic-buffer-pg:BUFFER_PG": {
+                "BUFFER_PG_LIST": [
+                {
+                    "port": "Ethernet4",
+                    "pg_num": "3",
+                    "profile": "NULL"
+                }
+                ]
+            }
+        }
+    },
     "BUFFER_PG_WRONG_PROFILE_VALUE": {
         "sonic-port:sonic-port": {
             "sonic-port:PORT": {

src/sonic-yang-models/yang-models/sonic-buffer-pg.yang

@@ -2,9 +2,7 @@ module sonic-buffer-pg {
     namespace "http://github.com/Azure/sonic-buffer-pg";
     prefix bpg;
 
-    import sonic-extension {
+    yang-version 1.1;
-        prefix sonic-ext;
-    }
 
     import sonic-port {
         prefix prt;
@@ -54,8 +52,13 @@ module sonic-buffer-pg {
 
                 leaf profile {
                     default 0;
-                    type leafref {
+                    type union {
-                        path "/bpf:sonic-buffer-profile/bpf:BUFFER_PROFILE/bpf:BUFFER_PROFILE_LIST/bpf:name";
+                        type leafref {
+                            path "/bpf:sonic-buffer-profile/bpf:BUFFER_PROFILE/bpf:BUFFER_PROFILE_LIST/bpf:name";
+                        }
+                        type string {
+                            pattern "NULL";
+                        }
                     }
                     description "Buffer Profile associated with Priority Group number for a port";
                 }