[Synthetics] Notify user when no previous run exists

[justinkambic]

Is your feature request related to a problem? Please describe.
Today, we provide the user with a screenshot of the last successful check in the Synthetics step view:

Describe the solution you'd like
When there is not a previously-successful journey, this area of the UI is devoid of any information:

Describe alternatives you've considered
We should provide an empty state for this, notifying them that no previous checks have succeeded and we therefore have no screenshot available for them to view.

Additional context
N/A

[paulb-elastic]

We might need to be careful with the wording, as there could be other reasons for no "last successful" image. With options for images not to be captured at all, or only on bad, there may have been successful runs, it’s just we don’t have images for them. Similarly, depending on the purge time for screenshots, the last successful run may have since had the screenshots purged.

Depending on how expensive it is to query for this information, maybe we can customise the message depending on if there have been successful runs, however we don’t have screenshots for them, compared with there being no successful runs.

If it is too inefficient to find this out, then we should probably use a more generic message, or point to several possible reasons.

[paulb-elastic]

Also linking to #330 as it's in a related area.

[justinkambic]

there could be other reasons for no "last successful" image.

We actually query for the last successful step before getting the screenshot for it already, so there is probably no concern with performance. We should definitely make sure all the possible states are covered; last successful step without a snapshot should appear too.