[SIEM] When no `siem:defaultIndex` is configured, 'SIEM job fetch failure' error shouldn't be displayed

[spong]

When no siem:defaultIndex is configured and we're showing the placeholder overview for each page, we shouldn't show the SIEM job fetch failure error as it is not helpful to the user, and adding indices as the page suggest will resolve the issue.

E.g.

[elasticmachine]

Pinging @elastic/siem (Team:SIEM)

[spong]

@MadameSheema this behavior is still present on 7.9.0-snapshot, albeit a little different... 😅