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/test/java/org/springframework/http/server/reactive/HeadersAdaptersTests.java

@@ -82,6 +82,19 @@ 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\"";