Contribute even when a sender makes no change

A pure consolidation does not suffer from Unnecessary Input Heuristic.
A Payjoin consolidation without sender change still breaks Common Input
Heuristic. Allow it.

payjoin/src/receive/mod.rs

@@ -361,8 +361,9 @@ impl ProvisionalProposal {
             return Err(SelectionError::from(InternalSelectionError::Empty));
         }
 
-        if self.payjoin_psbt.outputs.len() != 2 {
-            // Current UIH techniques only support many-input, two-output transactions.
+        if self.payjoin_psbt.outputs.len() >= 2 {
+            // This UIH avoidance function supports only
+            // many-input, n-output transactions such that n <= 2 for now
             return Err(SelectionError::from(InternalSelectionError::TooManyOutputs));
         }