Fix a NPE in proxied suspending functions

Closes gh-31809

Author: sdeleuze

spring-aop/src/main/java/org/springframework/aop/framework/CoroutinesUtils.java

@@ -38,8 +38,8 @@ static Object asFlow(Object publisher) {
 
 	@Nullable
 	@SuppressWarnings({"unchecked", "rawtypes"})
-	static Object awaitSingleOrNull(Object value, Object continuation) {
-		return MonoKt.awaitSingleOrNull(value instanceof Mono mono ? mono : Mono.just(value),
+	static Object awaitSingleOrNull(@Nullable Object value, Object continuation) {
+		return MonoKt.awaitSingleOrNull(value instanceof Mono mono ? mono : Mono.justOrEmpty(value),
 				(Continuation<Object>) continuation);
 	}
 

spring-aop/src/test/kotlin/org/springframework/aop/framework/CoroutinesUtilsTests.kt

@@ -0,0 +1,75 @@
+/*
+ * 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.aop.framework
+
+import kotlinx.coroutines.CoroutineName
+import kotlinx.coroutines.flow.Flow
+import kotlinx.coroutines.flow.toList
+import kotlinx.coroutines.runBlocking
+import org.assertj.core.api.Assertions.assertThat
+import org.junit.jupiter.api.Test
+import reactor.core.publisher.Flux
+import reactor.core.publisher.Mono
+import kotlin.coroutines.Continuation
+
+/**
+ * Tests for [CoroutinesUtils].
+ *
+ * @author Sebastien Deleuze
+ */
+class CoroutinesUtilsTests {
+
+	@Test
+	fun awaitSingleNonNullValue() {
+		val value = "foo"
+		val continuation = Continuation<Any>(CoroutineName("test")) { }
+		runBlocking {
+			assertThat(CoroutinesUtils.awaitSingleOrNull(value, continuation)).isEqualTo(value)
+		}
+	}
+
+	@Test
+	fun awaitSingleNullValue() {
+		val value = null
+		val continuation = Continuation<Any>(CoroutineName("test")) { }
+		runBlocking {
+			assertThat(CoroutinesUtils.awaitSingleOrNull(value, continuation)).isNull()
+		}
+	}
+
+	@Test
+	fun awaitSingleMonoValue() {
+		val value = "foo"
+		val continuation = Continuation<Any>(CoroutineName("test")) { }
+		runBlocking {
+			assertThat(CoroutinesUtils.awaitSingleOrNull(Mono.just(value), continuation)).isEqualTo(value)
+		}
+	}
+
+	@Test
+	@Suppress("UNCHECKED_CAST")
+	fun flow() {
+		val value1 = "foo"
+		val value2 = "bar"
+		val values = Flux.just(value1, value2)
+		val flow = CoroutinesUtils.asFlow(values) as Flow<String>
+		runBlocking {
+			assertThat(flow.toList()).containsExactly(value1, value2)
+		}
+	}
+
+}