lnworker.suggest_splits: (fix) don't force splitting

lnworker.suggest_splits for non-trampoline case tries to split amts over 5000 sat
but mpp_split.suggest_splits does not return splits where any part is smaller than 10000 sat.
So in practice, without trampoline, invoices between 5k and ~20k sats could not be paid.
This suggests that the logic should not be scattered in multiple places but merged into mpp_split.py...
This commit just does a quick fix though, to try again without splitting if there was no solution.

electrum/lnworker.py

@@ -1890,13 +1890,20 @@ class LNWallet(LNWorker):
                 if (amount_msat / final_total_msat > self.MPP_SPLIT_PART_FRACTION
                         and amount_msat > self.MPP_SPLIT_PART_MINAMT_MSAT):
                     exclude_single_part_payments = True
-        split_configurations = suggest_splits(
-            amount_msat,
-            channels_with_funds,
-            exclude_single_part_payments=exclude_single_part_payments,
-            exclude_multinode_payments=exclude_multinode_payments,
-            exclude_single_channel_splits=exclude_single_channel_splits
-        )
+
+        def get_splits():
+            return suggest_splits(
+                amount_msat,
+                channels_with_funds,
+                exclude_single_part_payments=exclude_single_part_payments,
+                exclude_multinode_payments=exclude_multinode_payments,
+                exclude_single_channel_splits=exclude_single_channel_splits
+            )
+
+        split_configurations = get_splits()
+        if not split_configurations and exclude_single_part_payments:
+            exclude_single_part_payments = False
+            split_configurations = get_splits()
         self.logger.info(f'suggest_split {amount_msat} returned {len(split_configurations)} configurations')
         return split_configurations