fix a bug with asm ingress gateway

Author: deveshdama

src/aks-preview/azext_aks_preview/managed_cluster_decorator.py

@@ -2205,6 +2205,8 @@ def _handle_egress_gateways_asm(self, new_profile: ServiceMeshProfile) -> Tuple[
             # if a gateway is enabled, enable the mesh
             if enable_egress_gateway:
                 new_profile.mode = CONST_AZURE_SERVICE_MESH_MODE_ISTIO
+                if new_profile.istio is None:
+                    new_profile.istio = self.models.IstioServiceMesh()  # pylint: disable=no-member
                 updated = True
 
             # ensure necessary fields
@@ -2259,6 +2261,8 @@ def _handle_ingress_gateways_asm(self, new_profile: ServiceMeshProfile) -> Tuple
             # if an ingress gateway is enabled, enable the mesh
             if enable_ingress_gateway:
                 new_profile.mode = CONST_AZURE_SERVICE_MESH_MODE_ISTIO
+                if new_profile.istio is None:
+                    new_profile.istio = self.models.IstioServiceMesh()  # pylint: disable=no-member
                 updated = True
 
             if not ingress_gateway_type: