Backport of fix a panic in the CLI when deleting an acl policy with an unknown name into release/1.16.x (#19691)

* backport of commit e07f4da212b2957e90550b7de11f6a783e0c374b

* backport of commit eecf61b67f4bfe14007d3057ed174fe7c185fac9

---------

Co-authored-by: Dhia Ayachi <dhia@hashicorp.com>

.changelog/19679.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+CLI: fix a panic when deleting a non existing policy by name.
+```

command/acl/acl_helpers.go

@@ -102,6 +102,10 @@ func GetPolicyIDByName(client *api.Client, name string) (string, error) {
 		return "", err
 	}
 
+	if policy == nil {
+		return "", fmt.Errorf("No such policy with name: %s", name)
+	}
+
 	return policy.ID, nil
 }
 

command/acl/acl_test.go

@@ -48,6 +48,36 @@ func Test_GetPolicyIDByName_Builtins(t *testing.T) {
 	}
 }
 
+func Test_GetPolicyIDByName_NotFound(t *testing.T) {
+	t.Parallel()
+
+	a := agent.StartTestAgent(t,
+		agent.TestAgent{
+			LogOutput: io.Discard,
+			HCL: `
+				primary_datacenter = "dc1"
+				acl {
+					enabled = true
+					tokens {
+						initial_management = "root"
+					}
+				}
+			`,
+		},
+	)
+
+	defer a.Shutdown()
+	testrpc.WaitForTestAgent(t, a.RPC, "dc1", testrpc.WithToken("root"))
+
+	client := a.Client()
+	client.AddHeader("X-Consul-Token", "root")
+
+	id, err := GetPolicyIDByName(client, "not_found")
+	require.Error(t, err)
+	require.Equal(t, "", id)
+
+}
+
 func Test_GetPolicyIDFromPartial_Builtins(t *testing.T) {
 	t.Parallel()