`Netty4HeadersAdapter.remove` returns empty list instead of `null` for non-existing key

[kzander91]

Netty4HeadersAdapter implements Map, therefore calling remove("unknown key") should return null, but it doesn't.
The reason is that the method first delegates to io.netty.handler.codec.http.HttpHeaders#getAll(java.lang.String) to get any existing values:

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

Lines 141 to 149 in 02cdd36

	@Override
	public @Nullable List<String> remove(Object key) {
	if (key instanceof String headerName) {
	List<String> previousValues = this.headers.getAll(headerName);
	this.headers.remove(headerName);
	return previousValues;
	}
	return null;
	}

Netty however returns an empty list instead of null for missing keys:
https://github.com/netty/netty/blob/a853a399b5dc976b70d457953741419db0cd45fa/codec-http/src/main/java/io/netty/handler/codec/http/HttpHeaders.java#L1282-L1292

Here's a reproducer:

import io.netty.handler.codec.http.DefaultHttpHeaders;
import io.netty.handler.codec.http.HttpHeaders;
import org.junit.jupiter.api.Test;
import org.springframework.http.support.Netty4HeadersAdapter;

import static org.assertj.core.api.Assertions.assertThat;

class Demo22ApplicationTests {

    @Test
    void reproducer() {
        HttpHeaders nettyHeaders = new DefaultHttpHeaders()
                .add("known", "value1")
                .add("known", "value2");
        Netty4HeadersAdapter adapter = new Netty4HeadersAdapter(nettyHeaders);
        assertThat(adapter.remove("known")).containsExactly("value1", "value2"); // succeeds
        assertThat(adapter.remove("unknown")).isNull(); // fails with `expected: null but was []`
    }

}