TomcatHttpHandlerAdapter continues after 0 bytes

This commit makes sure that TomcatServerHttpRequest::readFromInputStream
follows the same contract as the method it overrides, and returns
AbstractListenerReadPublisher.EMPTY_BUFFER when 0 bytes are read.

See gh-28241

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

@@ -153,7 +153,7 @@ else if (read == -1) {
 					return EOF_BUFFER;
 				}
 				else {
-					return null;
+					return AbstractListenerReadPublisher.EMPTY_BUFFER;
 				}
 			}
 			finally {