Fix pathExtension null-safety in Kotlin DSLs

Closes gh-32254

spring-webflux/src/test/kotlin/org/springframework/web/reactive/function/server/CoRouterFunctionDslTests.kt

@@ -112,6 +112,15 @@ class CoRouterFunctionDslTests {
 				.verifyComplete()
 	}
 
+	@Test
+	fun pathExtension() {
+		val mockRequest = get("https://example.com/test.properties").build()
+		val request = DefaultServerRequest(MockServerWebExchange.from(mockRequest), emptyList())
+		StepVerifier.create(sampleRouter().route(request))
+			.expectNextCount(1)
+			.verifyComplete()
+	}
+
 	@Test
 	fun resource() {
 		val mockRequest = get("https://example.com/response2.txt").build()
@@ -324,6 +333,9 @@ class CoRouterFunctionDslTests {
 				null
 			}
 		}
+		GET("/**", pathExtension { it == "properties" }) {
+			ok().bodyValueAndAwait("foo=bar")
+		}
 		path("/baz", ::handle)
 		GET("/rendering") { RenderingResponse.create("index").buildAndAwait() }
 		add(otherRouter)

spring-webflux/src/test/kotlin/org/springframework/web/reactive/function/server/RouterFunctionDslTests.kt

@@ -110,6 +110,15 @@ class RouterFunctionDslTests {
 				.verifyComplete()
 	}
 
+	@Test
+	fun pathExtension() {
+		val mockRequest = get("https://example.com/test.properties").build()
+		val request = DefaultServerRequest(MockServerWebExchange.from(mockRequest), emptyList())
+		StepVerifier.create(sampleRouter().route(request))
+			.expectNextCount(1)
+			.verifyComplete()
+	}
+
 	@Test
 	fun resource() {
 		val mockRequest = get("https://example.com/response2.txt").build()
@@ -256,6 +265,9 @@ class RouterFunctionDslTests {
 				Mono.empty()
 			}
 		}
+		GET(pathExtension { it == "properties" }) {
+			ok().bodyValue("foo=bar")
+		}
 		path("/baz", ::handle)
 		GET("/rendering") { RenderingResponse.create("index").build() }
 		add(otherRouter)