Fix NPE when calling NettyHeadersAdapter.add()

Prior to this commit, the `NettyHeadersAdapter` would directly delegate
the `add()` and `set()` calls to the adapted
`io.netty.handler.codec.http.HttpHeaders`. This implementation rejects
`null` values with exceptions.

This commit aligns the behavior here with other implementations, by not
rejecting null values but simply ignoring them.

Fixes gh-26277
(cherry-picked from commit 83c19cd60e)

spring-web/src/main/java/org/springframework/http/server/reactive/NettyHeadersAdapter.java

@@ -54,7 +54,9 @@ class NettyHeadersAdapter implements MultiValueMap<String, String> {
 
 	@Override
 	public void add(String key, @Nullable String value) {
-		this.headers.add(key, value);
+		if (value != null) {
+			this.headers.add(key, value);
+		}
 	}
 
 	@Override
@@ -69,7 +71,9 @@ class NettyHeadersAdapter implements MultiValueMap<String, String> {
 
 	@Override
 	public void set(String key, @Nullable String value) {
-		this.headers.set(key, value);
+		if (value != null) {
+			this.headers.set(key, value);
+		}
 	}
 
 	@Override

spring-web/src/test/java/org/springframework/http/server/reactive/HeadersAdaptersTests.java

@@ -94,6 +94,14 @@ class HeadersAdaptersTests {
 		assertThat(headers.get("TestHeader").size()).isEqualTo(1);
 	}
 
+	@ParameterizedHeadersTest
+	void nullValuesShouldNotFail(String displayName, MultiValueMap<String, String> headers) {
+		headers.add("TestHeader", null);
+		assertThat(headers.getFirst("TestHeader")).isNull();
+		headers.set("TestHeader", null);
+		assertThat(headers.getFirst("TestHeader")).isNull();
+	}
+
 	@Retention(RetentionPolicy.RUNTIME)
 	@Target(ElementType.METHOD)
 	@ParameterizedTest(name = "[{index}] {0}")