Avoid generating export file when no saved objects are actually exported

[ymao1]

With the recently added ability to filter out items onExport, it is possible to initiate an export where all selected objects are filtered out. This results in an ndjson with a single line containing the number of objects excluded, example:

{"excludedObjects":[{"id":"2a16ac09-d834-11eb-b614-2b1b222c1719","reason":"excluded","type":"alert"}],"excludedObjectsCount":1,"exportedCount":0,"missingRefCount":0,"missingReferences":[]}

In this scenario, it would be nice to avoid generating this file altogether and show a warning to the user informing them that none of their objects were actually exported.

[elasticmachine]

Pinging @elastic/kibana-core (Team:Core)