Bug/admin: Global statistics for recently-created accounts

Global statistics computation became confused when presented with a user account which had a database entry for the first month in the statistics computation interval (currently 30 days), but in which the first weight entry was after the start of the interval. The code which computes the user's trend slope would pass undefined trend items to the fitter, generating a snowdrift of (otherwise harmless) warning messages. I added a test for undefined trend values returned by the aggregator, which causes the coverage of such users' logs to be deemed incomplete and thus excluded from the summary trend analysis.