Warn rather than throw on empty task

[pauby]

When there is an empty task found during import, warn rather than throw an exception. Taking a todo.txt as follows:

2018-04-04 Write a blog post
2018-04-04 +hello
2018-04-09 Investigate comms errors @computer

An exception will be thrown during import for the middle todo. This stops the whole import until it's manually fixed.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.