Empty string Event tag

While looking at #6142 I noticed in the [Grok filter](https://github.com/logstash-plugins/logstash-filter-grok/blob/565126116d55a8833810a926c4597f8c8188b9f2/lib/logstash/filters/grok.rb#L205) for example that it is possible to specify and empty string as a tag in the plugin config, then the plugin [does not verify for such a condition](https://github.com/logstash-plugins/logstash-filter-grok/blob/565126116d55a8833810a926c4597f8c8188b9f2/lib/logstash/filters/grok.rb#L306) when tagging.

I think we should probably just discard empty string tags at the [Event implementation](https://github.com/elastic/logstash/blob/ae60c7dfd44d32892f526c853dab1bde5705e370/logstash-core-event-java/src/main/java/org/logstash/Event.java#L347-L357) to avoid such a condition.