Handle no data case gracefully

This MR handles various corner cases that crop up when no data is available for this time span. This can show up in the online analysis during times of low/no locked data for an extended period of time.

This implements two of the items in the checklist in #64 (closed). The rest have either been implemented at this point or haven't been shown as issues. Separate issues may be opened again if they show up.

Closes #64 (closed).