Include original exception if cause is null

Closes gh-33080
See gh-32952

spring-web/src/main/java/org/springframework/http/client/ReactorNettyClientRequest.java

@@ -140,7 +140,7 @@ static IOException convertException(RuntimeException ex) {
 				return ioEx;
 			}
 		}
-		return new IOException(ex.getMessage(), cause);
+		return new IOException(ex.getMessage(), (cause != null ? cause : ex));
 	}