[Stack Monitoring] Add default value for source and target to prevent…

… runtime error (elastic#180043)

## 📓 Summary

The issue occurs because when requesting from the server the details
about elasticsearch for the cluster (`POST
/api/monitoring/v1/clusters/{clusterUuid}/elasticsearch`), it includes a
list of shard activities in the response.

Items of this `shardActivity`
([ElasticsearchIndexRecoveryShard](https://github.com/elastic/kibana/blob/main/x-pack/plugins/monitoring/common/types/es.ts#L380))
may have an **optional** `source` property, but once the data reaches
the client app, the current implementation gives for granted this value
will always exist, and when accessing specific properties of `source`,
this generates a runtime error, resulting in a blank screen.

This change uses a default empty `source` and `target` object on the
client to prevent a reference error when accessing their properties and
set them to undefined.

---------

Co-authored-by: Marco Antonio Ghiani <[email protected]>

x-pack/plugins/monitoring/public/components/elasticsearch/shard_activity/parse_props.js

@@ -35,8 +35,8 @@ export const parseProps = (props) => {
     total_time_in_millis: totalTimeInMillis,
     start_time: mbStartTime,
     total_time: mbTotalTime,
-    source,
-    target,
+    source = {}, // This property is potentially undefined due to a legacy search where `source` is optionally existing
+    target = {}, // This property is potentially undefined due to a legacy search where `target` is optionally existing
     translog,
     type,
     timezone,