Refine null-safety in more modules

This commit refines the null-safety in all remaining modules
except spring-test.

See gh-32475

spring-webmvc/src/main/java/org/springframework/web/servlet/function/DefaultServerRequest.java

@@ -378,6 +378,7 @@ class DefaultServerRequest implements ServerRequest {
 		}
 
 		@Override
+		@Nullable
 		public InetSocketAddress host() {
 			return this.httpHeaders.getHost();
 		}