Fix "remove" implementation in netty headers adapter

Prior to this commit, the `Netty4HeadersAdapter` `MultiValueMapi#remove`
implementation would return an empty list if no value was present. This
is not consistent with other implementations.

This change ensures that `null` is returned for those cases.

Fixes gh-36226

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

@@ -129,7 +129,7 @@ class UndertowHeadersAdapter implements MultiValueMap<String, String> {
 	@Override
 	@Nullable
 	public List<String> remove(Object key) {
-		if (key instanceof String headerName) {
+		if (key instanceof String headerName && this.headers.contains(headerName)) {
 			Collection<String> removed = this.headers.remove(headerName);
 			if (removed != null) {
 				return new ArrayList<>(removed);

spring-web/src/main/java/org/springframework/http/support/Netty4HeadersAdapter.java

@@ -144,7 +144,7 @@ public final class Netty4HeadersAdapter implements MultiValueMap<String, String>
 	@Nullable
 	@Override
 	public List<String> remove(Object key) {
-		if (key instanceof String headerName) {
+		if (key instanceof String headerName && this.headers.contains(headerName)) {
 			List<String> previousValues = this.headers.getAll(headerName);
 			this.headers.remove(headerName);
 			return previousValues;

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

@@ -85,6 +85,19 @@ class HeadersAdaptersTests {
 				.hasSize(0);
 	}
 
+	@ParameterizedHeadersTest
+	void shouldReturnNullWhenRemoveUnknown(MultiValueMap<String, String> headers) {
+		assertThat(headers.remove("unknown")).isEqualTo(null);
+	}
+
+	@ParameterizedHeadersTest
+	void shouldReturnPreviousValuesWhenRemove(MultiValueMap<String, String> headers) {
+		headers.add("TestHeader", "first");
+		headers.add("TestHEADER", "second");
+
+		assertThat(headers.remove("testheader")).hasSize(2);
+	}
+
 	@ParameterizedPopulatedHeadersTest
 	void toString(MultiValueMap<String, String> headers) {
 		String expectedFirstHeader = "TestHeader:\"first\", \"second\", \"third\"";