[fix]: support nil output in RenderTester Publisher extension (#236)

### Issue

- the `WorkflowCombine` `RenderTester` extensions didn't allow `nil` output

### Description

- add a new method to support `nil` output and match the naming convention of analogous methods
- deprecate old method
- update tests

addresses #235

WorkflowCombine/Testing/PublisherTesting.swift

@@ -23,16 +23,17 @@ import XCTest
 @testable import WorkflowCombine
 
 extension RenderTester {
-    /// Expect a `Publisher`s.
+    /// Expect a `Publisher`-based Workflow.
     ///
     /// `PublisherWorkflow` is used to subscribe to `Publisher`s.
     ///
     /// - Parameters:
+    ///   - publisher: Type of the Publisher-based Workflow to expect
     ///   - producingOutput: An output that should be returned when this worker is requested, if any.
     ///   - key: Key to expect this `Workflow` to be rendered with.
     public func expect<PublisherType: Publisher>(
         publisher: PublisherType.Type,
-        output: PublisherType.Output,
+        producingOutput output: PublisherType.Output? = nil,
         key: String = ""
     ) -> RenderTester<WorkflowType> where PublisherType.Failure == Never {
         expectWorkflow(
@@ -43,6 +44,19 @@ extension RenderTester {
             assertions: { _ in }
         )
     }
+
+    @available(*, deprecated, renamed: "expect(publisher:producingOutput:key:)")
+    public func expect<PublisherType: Publisher>(
+        publisher: PublisherType.Type,
+        output: PublisherType.Output,
+        key: String = ""
+    ) -> RenderTester<WorkflowType> where PublisherType.Failure == Never {
+        expect(
+            publisher: publisher,
+            producingOutput: output,
+            key: key
+        )
+    }
 }
 
 #endif

WorkflowCombine/TestingTests/PublisherTests.swift

@@ -16,10 +16,26 @@ class PublisherTests: XCTestCase {
     func testPublisherWorkflow() {
         TestWorkflow()
             .renderTester()
-            .expect(publisher: Publishers.Sequence<[Int], Never>.self, output: 1, key: "123")
+            .expect(
+                publisher: Publishers.Sequence<[Int], Never>.self,
+                producingOutput: 1,
+                key: "123"
+            )
             .render {}
     }
 
+    func test_publisher_no_output() {
+        TestWorkflow()
+            .renderTester()
+            .expect(
+                publisher: Publishers.Sequence<[Int], Never>.self,
+                producingOutput: nil,
+                key: "123"
+            )
+            .render {}
+            .assertNoAction()
+    }
+
     struct TestWorkflow: Workflow {
         typealias State = Void
         typealias Rendering = Void