Fix zero-fee tipping and add edge case test

contracts/tipstream.clar

@@ -111,7 +111,10 @@
         (asserts! (not (default-to false (map-get? blocked-users { blocker: recipient, blocked: tx-sender }))) err-user-blocked)
 
         (try! (stx-transfer? net-amount tx-sender recipient))
-        (try! (stx-transfer? fee tx-sender (var-get contract-owner)))
+        (if (> fee u0)
+            (try! (stx-transfer? fee tx-sender (var-get contract-owner)))
+            true
+        )
 
         (map-set tips
             { tip-id: tip-id }

tests/tipstream.test.ts

@@ -420,6 +420,29 @@ describe("TipStream Contract Tests", () => {
             );
             expect(result).toBeErr(Cl.uint(100));
         });
+
+        it("sending a tip with zero fee transfers full amount to recipient", () => {
+            simnet.callPublicFn(
+                "tipstream",
+                "set-fee-basis-points",
+                [Cl.uint(0)],
+                deployer
+            );
+
+            const { result, events } = simnet.callPublicFn(
+                "tipstream",
+                "send-tip",
+                [Cl.principal(wallet2), Cl.uint(1000000), Cl.stringUtf8("No fee tip")],
+                wallet1
+            );
+
+            expect(result).toBeOk(Cl.uint(0));
+
+            const transfers = events.filter(e => e.event === "stx_transfer_event");
+            expect(transfers).toHaveLength(1);
+            expect(transfers[0].data.amount).toBe("1000000");
+            expect(transfers[0].data.recipient).toBe(wallet2);
+        });
     });
 
     describe("Batch Tipping", () => {