[FLINK-18998] No watermark is shown in Flink UI when ProcessingTime i…

…s used

Display more informative message stating that Watermarks are only available when using
event time if there are no watermarks to display in the web UI.

This closes apache#14483.

flink-runtime-web/web-dashboard/src/app/share/pipes/humanize-watermark.pipe.ts

@@ -25,7 +25,7 @@ import { LONG_MIN_VALUE } from 'config';
 export class HumanizeWatermarkPipe implements PipeTransform {
  transform(value: any): any {
  if (isNaN(value) || value <= LONG_MIN_VALUE) {
- return 'No Watermark';
+ return 'No Watermark (Watermarks are only available if EventTime is used)';
  } else {
  return value;
  }