fix: use the validStart null flag in TransactionId.compareTo

The validStart null-handling branch returned based on thisAccountIdIsNull instead of thisStartIsNull. Since that branch is only reachable when both ids agree on accountId nullness, the comparison returned the same sign in both directions, violating the Comparable antisymmetry and transitivity contracts.

Fixes #2740

Author: the2015

sdk/src/main/java/com/hedera/hashgraph/sdk/TransactionId.java

@@ -529,7 +529,7 @@ public int compareTo(TransactionId o) {
         var thisStartIsNull = (validStart == null);
         var otherStartIsNull = (o.validStart == null);
         if (thisStartIsNull != otherStartIsNull) {
-            return thisAccountIdIsNull ? -1 : 1;
+            return thisStartIsNull ? -1 : 1;
         }
         if (!thisStartIsNull) {
             return validStart.compareTo(o.validStart);

sdk/src/test/java/com/hedera/hashgraph/sdk/TransactionIdTest.java

@@ -186,4 +186,22 @@ void equalsHashCodeContractWithNonce() {
                 txnId1.equals(txnId2) && txnId1.hashCode() != txnId2.hashCode(),
                 "equals/hashCode contract violation: equal objects must have same hashCode");
     }
+
+    @Test
+    void compareToIsAntisymmetricWhenOnlyOneValidStartIsNull() {
+        var accountId = AccountId.fromString("0.0.23847");
+        var withValidStart = new TransactionId(accountId, Instant.ofEpochSecond(1588539964));
+        var withoutValidStart = new TransactionId(accountId, null);
+
+        // A null validStart must sort before a non-null one no matter which side it is on.
+        assertThat(withoutValidStart.compareTo(withValidStart)).isEqualTo(-1);
+        assertThat(withValidStart.compareTo(withoutValidStart)).isEqualTo(1);
+
+        // Same requirement when neither transaction id carries an account id.
+        var noAccountWithStart = new TransactionId(null, Instant.ofEpochSecond(1588539964));
+        var noAccountWithoutStart = new TransactionId(null, null);
+
+        assertThat(noAccountWithoutStart.compareTo(noAccountWithStart)).isEqualTo(-1);
+        assertThat(noAccountWithStart.compareTo(noAccountWithoutStart)).isEqualTo(1);
+    }
 }