Add awaitBodyOrNull to WebClient.ResponseSpec

Such variant is already provided for ClientResponse but not
for WebClient.ResponseSpec.

Closes gh-26731

spring-webflux/src/main/kotlin/org/springframework/web/reactive/function/client/WebClientExtensions.kt

@@ -21,6 +21,7 @@ import kotlinx.coroutines.flow.Flow
 import kotlinx.coroutines.reactive.asFlow
 import kotlinx.coroutines.reactive.awaitSingle
 import kotlinx.coroutines.reactive.awaitSingle
+import kotlinx.coroutines.reactive.awaitSingleOrNull
 import kotlinx.coroutines.reactor.asFlux
 import kotlinx.coroutines.reactor.mono
 import org.reactivestreams.Publisher
@@ -143,6 +144,17 @@ suspend inline fun <reified T : Any> WebClient.ResponseSpec.awaitBody() : T =
 		else -> bodyToMono<T>().awaitSingle()
 	}
 
+/**
+ * Coroutines variant of [WebClient.ResponseSpec.bodyToMono].
+ *
+ * @author Valentin Shakhov
+ */
+suspend inline fun <reified T : Any> WebClient.ResponseSpec.awaitBodyOrNull() : T? =
+	when (T::class) {
+		Unit::class -> awaitBodilessEntity().let { Unit as T? }
+		else -> bodyToMono<T>().awaitSingleOrNull()
+	}
+
 /**
  * Coroutines variant of [WebClient.ResponseSpec.toBodilessEntity].
  */

spring-webflux/src/test/kotlin/org/springframework/web/reactive/function/client/WebClientExtensionsTests.kt

@@ -136,6 +136,25 @@ class WebClientExtensionsTests {
 		}
 	}
 
+	@Test
+	fun awaitBodyOrNull() {
+		val spec = mockk<WebClient.ResponseSpec>()
+		every { spec.bodyToMono<String>() } returns Mono.just("foo")
+		runBlocking {
+			assertThat(spec.awaitBodyOrNull<String>()).isEqualTo("foo")
+		}
+	}
+
+	@Test
+	fun `awaitBodyOrNull of type Unit`() {
+		val spec = mockk<WebClient.ResponseSpec>()
+		val entity = mockk<ResponseEntity<Void>>()
+		every { spec.toBodilessEntity() } returns Mono.just(entity)
+		runBlocking {
+			assertThat(spec.awaitBodyOrNull<Unit>()).isEqualTo(Unit)
+		}
+	}
+
 	@Test
 	fun awaitBodilessEntity() {
 		val spec = mockk<WebClient.ResponseSpec>()