Allow null id/event in ServerSentEvent

The builder itself does not allow `null` values so this shouldn't be
necessary, but because the offending change was introduced late in the
6.2.x line, we'll be more flexible here.

Fixes gh-36634

spring-web/src/main/java/org/springframework/http/codec/ServerSentEvent.java

@@ -267,8 +267,10 @@ public final class ServerSentEvent<T> {
 		}
 
 		private static void checkEvent(String content) {
-			Assert.isTrue(content.indexOf('\n') == -1 && content.indexOf('\r') == -1,
-					"illegal character '\\n' or '\\r' in event content");
+			if (content != null) {
+				Assert.isTrue(content.indexOf('\n') == -1 && content.indexOf('\r') == -1,
+						"illegal character '\\n' or '\\r' in event content");
+			}
 		}
 
 		@Override

spring-web/src/test/java/org/springframework/http/codec/ServerSentEventTests.java

@@ -18,10 +18,12 @@ package org.springframework.http.codec;
 
 import java.util.stream.Stream;
 
+import org.junit.jupiter.api.Test;
 import org.junit.jupiter.params.ParameterizedTest;
 import org.junit.jupiter.params.provider.Arguments;
 import org.junit.jupiter.params.provider.MethodSource;
 
+import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;
 
 /**
@@ -44,6 +46,18 @@ class ServerSentEventTests {
 				ServerSentEvent.<String>builder().event("first" + newLine + "second").build());
 	}
 
+	@Test // gh-36634
+	void allowsNullId() {
+		ServerSentEvent<String> event = ServerSentEvent.<String>builder().id(null).event("first").build();
+		assertThat(event.format()).contains("event:first").doesNotContain("id");
+	}
+
+	@Test // gh-36634
+	void allowsNullEvent() {
+		ServerSentEvent<String> event = ServerSentEvent.<String>builder().id("42").event(null).build();
+		assertThat(event.format()).contains("id:42").doesNotContain("event");
+	}
+
 	private static Stream<Arguments> newLineCharacters() {
 		return Stream.of(
 				Arguments.of("\n", "LF"),