update_agent: handle new output format for loginctl

In systemd 254 the `loginctl list-sessions --json` output was modified slightly:

Before
```
[{"session":"4","uid":1000,"user":"core","seat":"","tty":""}]
```

After
```
[{"session":"17","uid":1000,"user":"core","seat":"","tty":"n/a","state":"active","idle":false,"since":null}]
```

Notice the change from `""` to `"n/a"` for the `tty` field.

This change probably happened in one of:

- systemd/systemd#27769
- systemd/systemd#27740
- systemd/systemd#27606

Let's update the code to handle the new case and rename the function to
be a little more appropriate since the use case for it is narrow.

Fixes coreos/fedora-coreos-tracker#1547

src/update_agent/mod.rs

@@ -68,7 +68,7 @@ lazy_static::lazy_static! {
 #[derive(Debug, Deserialize)]
 pub struct SessionJson {
     user: String,
-    #[serde(deserialize_with = "empty_string_as_none")]
+    #[serde(deserialize_with = "deserialize_systemd_tty_canonicalized")]
     tty: Option<String>,
 }
 
@@ -79,14 +79,16 @@ pub struct InteractiveSession {
     tty_dev: String,
 }
 
-/// Function to deserialize field to `Option<String>`, where empty strings are
-/// deserialized into `None`.
-fn empty_string_as_none<'de, D>(deserializer: D) -> Result<Option<String>, D::Error>
+/// Function to deserialize field to `Option<String>`, where empty strings or
+/// `n/a` (not available) strings are deserialized into `None`. In systemd v254+
+/// loginctl list-sessions --json started outputting `n/a` instead of an empty
+/// string for tty.
+fn deserialize_systemd_tty_canonicalized<'de, D>(deserializer: D) -> Result<Option<String>, D::Error>
 where
     D: Deserializer<'de>,
 {
     let s = String::deserialize(deserializer)?;
-    if s.is_empty() {
+    if s.is_empty() || s == "n/a" {
         Ok(None)
     } else {
         Ok(Some(s))