Fix NPE on resps with a single epoch where finding the closest epoch was necessary

The file has an epoch at the start without an end time specified. This does not generally cause issues with sensor test suite because the epochs are presented to the user for selection, but as we saw in a recent ticket does cause issues with ITT's interface into it, because resp epochs aren't presented to the user before running the test. This prevents a NPE that occurs with the data in the new test case, by adding an explicit check for null epoch ends.