Jetpack Scan: improve last scanned date display when no date exists

[v18]

When there is no last scanned date, show 'Queued for scanning'. Here's what it looks like now:

There is no date shown at all. Here's what it looks like when there is a data to display:

[designsimply]

Closing this as it should theoretically be an edge case when no scans have happened yet, and the extra information to note that scanning is queued can probably be inferred.