Revert "Work around Framework regression in handling of null model values"

This reverts commit 5fc67c5.

Closes gh-16991

Author: wilkinsona

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/reactive/error/DefaultErrorAttributes.java

@@ -83,8 +83,7 @@ public Map<String, Object> getErrorAttributes(ServerRequest request, boolean inc
 		HttpStatus errorStatus = determineHttpStatus(error);
 		errorAttributes.put("status", errorStatus.value());
 		errorAttributes.put("error", errorStatus.getReasonPhrase());
-		String message = determineMessage(error);
-		errorAttributes.put("message", (message != null) ? message : "");
+		errorAttributes.put("message", determineMessage(error));
 		handleException(errorAttributes, determineException(error), includeStackTrace);
 		return errorAttributes;
 	}