Fix pathExtension null-safety in Kotlin DSLs

Closes gh-32254

spring-webmvc/src/main/kotlin/org/springframework/web/servlet/function/RouterFunctionDsl.kt

@@ -564,7 +564,7 @@ class RouterFunctionDsl internal constructor (private val init: (RouterFunctionD
 	 * Route to the given handler function if the given pathExtension predicate applies.
 	 * @see RouterFunctions.route
 	 */
-	fun pathExtension(predicate: (String) -> Boolean, f: (ServerRequest) -> ServerResponse) {
+	fun pathExtension(predicate: (String?) -> Boolean, f: (ServerRequest) -> ServerResponse) {
 		builder.add(RouterFunctions.route(RequestPredicates.pathExtension(predicate), HandlerFunction(f)))
 	}
 
@@ -573,7 +573,7 @@ class RouterFunctionDsl internal constructor (private val init: (RouterFunctionD
 	 * predicate.
 	 * @see RequestPredicates.pathExtension
 	 */
-	fun pathExtension(predicate: (String) -> Boolean): RequestPredicate =
+	fun pathExtension(predicate: (String?) -> Boolean): RequestPredicate =
 			RequestPredicates.pathExtension(predicate)
 
 	/**

spring-webmvc/src/test/kotlin/org/springframework/web/servlet/function/RouterFunctionDslTests.kt

@@ -88,6 +88,13 @@ class RouterFunctionDslTests {
 		assertThat(sampleRouter().route(request).isPresent).isTrue()
 	}
 
+	@Test
+	fun pathExtension() {
+		val servletRequest = PathPatternsTestUtils.initRequest("GET", "/test.properties", true)
+		val request = DefaultServerRequest(servletRequest, emptyList())
+		assertThat(sampleRouter().route(request).isPresent).isTrue()
+	}
+
 	@Test
 	fun resource() {
 		val servletRequest = PathPatternsTestUtils.initRequest("GET","/response2.txt", true)
@@ -185,6 +192,9 @@ class RouterFunctionDslTests {
 				null
 			}
 		}
+		GET(pathExtension { it == "properties" }) {
+			ok().body("foo=bar")
+		}
 		path("/baz", ::handle)
 		GET("/rendering") { RenderingResponse.create("index").build() }
 		add(otherRouter)