Explicitly use "" for KEY within the tests

Causes the provided keystore to be used over our keystore.

substrate/node/src/keystore.rs

@@ -8,6 +8,9 @@ pub struct Keystore(sr25519::Pair);
 impl Keystore {
   pub fn from_env() -> Option<Self> {
     let mut key_hex = serai_env::var("KEY")?;
+    if key_hex.is_empty() {
+      None?;
+    }
     let mut key = hex::decode(&key_hex).expect("KEY from environment wasn't hex");
     key_hex.zeroize();