Don't redraw scan chart before first scan

[argilo]

If "Show Scan Plot" is toggled on before the first scan has completed, or the scan plot is open and the user navigates away from the Auto-RX tab and back before the first scan has completed, then an error occurs and the scan plot renders incorrectly:

This happens because scan_chart_latest_timestamp is not defined until the first scan has completed, but redraw_scan_chart (which is invoked when the scan chart is opened, or when the user navigates back from another tab) attempts to use it anyway.

To fix the problem, I've added a check so that redraw_scan_chart exits early if scan_chart_latest_timestamp is not yet defined.