Allow resolution of optional HandlerMethod

Closes gh-35067

Author: rstoyanchev

spring-web/src/main/java/org/springframework/web/method/support/InvocableHandlerMethod.java

@@ -217,6 +217,10 @@ public void setMethodValidator(@Nullable MethodValidator methodValidator) {
 			if (args[i] != null) {
 				continue;
 			}
+			if (parameter.getParameterType().equals(HandlerMethod.class) && parameter.isOptional()) {
+				args[i] = null;
+				continue;
+			}
 			if (!this.resolvers.supportsParameter(parameter)) {
 				throw new IllegalStateException(formatArgumentError(parameter, "No suitable resolver"));
 			}

spring-web/src/test/java/org/springframework/web/method/support/InvocableHandlerMethodTests.java

@@ -18,13 +18,15 @@
 
 import java.lang.reflect.Method;
 
+import org.jspecify.annotations.Nullable;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 
 import org.springframework.core.MethodParameter;
 import org.springframework.web.bind.support.WebDataBinderFactory;
 import org.springframework.web.context.request.NativeWebRequest;
 import org.springframework.web.context.request.ServletWebRequest;
+import org.springframework.web.method.HandlerMethod;
 import org.springframework.web.testfixture.method.ResolvableMethod;
 import org.springframework.web.testfixture.servlet.MockHttpServletRequest;
 import org.springframework.web.testfixture.servlet.MockHttpServletResponse;
@@ -105,6 +107,14 @@ void resolveProvidedArgFirst() throws Exception {
 		assertThat(value).isEqualTo("2-value2");
 	}
 
+	@Test
+	void resolveHandlerMethodArgToNull() throws Exception {
+		Object value = getInvocable(HandlerMethod.class).invokeForRequest(request, null);
+
+		assertThat(value).isNotNull();
+		assertThat(value).isEqualTo("success");
+	}
+
 	@Test
 	void exceptionInResolvingArg() {
 		this.composite.addResolver(new ExceptionRaisingArgumentResolver());
@@ -184,6 +194,10 @@ public void handle(double amount) {
 		public void handleWithException(Throwable ex) throws Throwable {
 			throw ex;
 		}
+
+		public String handleHandlerMethod(@Nullable HandlerMethod handlerMethod) {
+			return "success";
+		}
 	}