[FEATURE] Provide empty states for Overview page

[kamingleung]

Is your feature request related to a problem?
Issues today: On the overview page, there are no clear messages or guidance on what to do when there are no findings and alerts found within the time range.
Current behavior:

1. It shows a blank chart
2. The count of Total active alerts and Total findings are in loading state (they kept on blinking)

What solution would you like?
Once the UI had finished fetching the data, and if there are no alerts AND no findings, we should replace the chart with an empty prompt:

• Header: No alerts and findings found
• Description: Adjust the time range to see more results or create a detector to generate findings.
• Clicking on the link create a detector will go to the Create detector page
• Use OUI empty prompt component

Once the UI had finished fetching the data with no alerts or no findings, we should show 0 for Total alerts trigger or Total findings respectively:

• Use OUI stat component with subdued color when the count is 0.
• It won't be a link if the value is 0.

We will also need to update the other empty states on the Overview page with similar messaging:

Most frequent detection rules:
No findings with detection rules. Adjust the time range to see more results.

Recent alerts:
No recent alerts. Adjust the time range to see more results.

Recent findings:
No recent findings. Adjust the time range to see more results.

[amsiglan]

@kamingleung Should those counts still be links to the Alerts/Findings pages or should they be disabled?

[amsiglan]

@kamingleung Should we have similar empty prompts in the Findings and Alerts pages as well?

[amsiglan]

@kamingleung We also need some text for the most frequent rules widget when it is empty

[kamingleung]

@amsiglan

Should those counts still be links to the Alerts/Findings pages or should they be disabled?

^ Yes, they won't be clickable when the value is 0.

We also need some text for the most frequent rules widget when it is empty

^ The empty state text would be
No findings with detection rules. Adjust the time range to see more results.

We will also need to update the other empty states on the Overview page with similar messaging:

Recent alerts:
No recent alerts. Adjust the time range to see more results.

Recent findings:
No recent findings. Adjust the time range to see more results.

[kamingleung]

@amsiglan

Should we have similar empty prompts in the Findings and Alerts pages as well?

A separate issue was created for these two pages: #471