Refine ModelAttributeMethodProcessor Kotlin exception handling

This commit refines ModelAttributeMethodProcessor Kotlin exception
handling in order to throw a proper MethodArgumentNotValidException
instead of a NullPointerException when Kotlin null-safety constraints
are not fulfilled, translating to an HTTP error with 400 status code
(Bad Request) instead of 500 (Internal Server Error).

Closes gh-23846

spring-web/src/test/kotlin/org/springframework/web/method/annotation/ModelAttributeMethodProcessorKotlinTests.kt

@@ -0,0 +1,87 @@
+/*
+ * Copyright 2002-2023 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.web.method.annotation
+
+import org.assertj.core.api.Assertions
+import org.junit.jupiter.api.BeforeEach
+import org.junit.jupiter.api.Test
+import org.mockito.ArgumentMatchers.*
+import org.mockito.BDDMockito
+import org.mockito.Mockito
+import org.springframework.core.MethodParameter
+import org.springframework.core.annotation.SynthesizingMethodParameter
+import org.springframework.web.bind.MethodArgumentNotValidException
+import org.springframework.web.bind.support.WebDataBinderFactory
+import org.springframework.web.bind.support.WebRequestDataBinder
+import org.springframework.web.context.request.ServletWebRequest
+import org.springframework.web.method.support.ModelAndViewContainer
+import org.springframework.web.testfixture.servlet.MockHttpServletRequest
+import kotlin.annotation.AnnotationTarget.*
+
+/**
+ * Kotlin test fixture for [ModelAttributeMethodProcessor].
+ *
+ * @author Sebastien Deleuze
+ */
+class ModelAttributeMethodProcessorKotlinTests {
+
+	private lateinit var container: ModelAndViewContainer
+
+	private lateinit var processor: ModelAttributeMethodProcessor
+
+	private lateinit var param: MethodParameter
+
+
+	@BeforeEach
+	fun setup() {
+		container = ModelAndViewContainer()
+		processor = ModelAttributeMethodProcessor(false)
+		var method = ModelAttributeHandler::class.java.getDeclaredMethod("test",Param::class.java)
+		param = SynthesizingMethodParameter(method, 0)
+	}
+
+	@Test
+	fun resolveArgumentWithValue() {
+		val mockRequest = MockHttpServletRequest().apply { addParameter("a", "b") }
+		val requestWithParam = ServletWebRequest(mockRequest)
+		val factory = Mockito.mock<WebDataBinderFactory>()
+		BDDMockito.given(factory.createBinder(any(), any(), eq("param")))
+			.willAnswer { WebRequestDataBinder(it.getArgument(1)) }
+		Assertions.assertThat(processor.resolveArgument(this.param, container, requestWithParam, factory)).isEqualTo(Param("b"))
+	}
+
+	@Test
+	fun throwMethodArgumentNotValidExceptionWithNull() {
+		val mockRequest = MockHttpServletRequest().apply { addParameter("a", null) }
+		val requestWithParam = ServletWebRequest(mockRequest)
+		val factory = Mockito.mock<WebDataBinderFactory>()
+		BDDMockito.given(factory.createBinder(any(), any(), eq("param")))
+			.willAnswer { WebRequestDataBinder(it.getArgument(1)) }
+		Assertions.assertThatThrownBy {
+			processor.resolveArgument(this.param, container, requestWithParam, factory)
+		}.isInstanceOf(MethodArgumentNotValidException::class.java)
+			.hasMessageContaining("parameter a")
+	}
+
+	private data class Param(val a: String)
+
+	@Suppress("UNUSED_PARAMETER")
+	private class ModelAttributeHandler {
+		fun test(param: Param) { }
+	}
+
+}