ipc: ensure backward compatibility of the protocol

If the client was compiled with the previous version of the protocol,
initialize StartSession.env with the default value instead of failing to
deserialize the message.

greetd_ipc/src/lib.rs

@@ -75,7 +75,11 @@ pub enum Request {
 
     /// Start a successfully logged in session. This will fail if the session
     /// has pending messages or has encountered an error.
-    StartSession { cmd: Vec<String>, env: Vec<String> },
+    StartSession {
+        cmd: Vec<String>,
+        #[serde(default)]
+        env: Vec<String>,
+    },
 
     /// Cancel a session. This can only be done if the session has not been
     /// started. Cancel does not have to be called if an error has been