CoinChooser: avoid NotEnoughFunds if zero buckets are sufficient

closes #5752

Adapted from @JeremyRand's fix

electrum/tests/test_coinchooser.py

@@ -0,0 +1,20 @@
+from electrum.coinchooser import CoinChooserPrivacy
+from electrum.util import NotEnoughFunds
+
+from . import ElectrumTestCase
+
+
+class TestCoinChooser(ElectrumTestCase):
+
+    def test_bucket_candidates_with_empty_buckets(self):
+        def sufficient_funds(buckets, *, bucket_value_sum):
+            return True
+        coin_chooser = CoinChooserPrivacy()
+        self.assertEqual([[]], coin_chooser.bucket_candidates_any([], sufficient_funds))
+        self.assertEqual([[]], coin_chooser.bucket_candidates_prefer_confirmed([], sufficient_funds))
+        def sufficient_funds(buckets, *, bucket_value_sum):
+            return False
+        with self.assertRaises(NotEnoughFunds):
+            coin_chooser.bucket_candidates_any([], sufficient_funds)
+        with self.assertRaises(NotEnoughFunds):
+            coin_chooser.bucket_candidates_prefer_confirmed([], sufficient_funds)