fix: prevent crash in Chart.Render when repository has no commits

[JC-Chung]

Opening the Statistics view in an empty repository (no commits) crashes
with ArgumentOutOfRangeException in NextSampleTime.

When there are no commits, StatisticsReport._minSampleTime stays at
DateTime.MaxValue and _maxSampleTime stays at DateTime.MinValue.
For StatisticsMode.All, Count is calculated as
(end.Year - start.Year) * 12 + ..., which yields a large negative number.
The guard samples.Count == 0 in Chart.Render does not catch negative
values, so rendering proceeds with time = DateTime.MinValue (year 1,
month 1). NextSampleTime then tries new DateTime(0, 12, 1), which
throws ArgumentOutOfRangeException.

Fix by changing samples.Count == 0 to samples.Count <= 0.