Handle empty statistics (#482)

This prevents most of the fields from being set to NaN.

src/Numerics/Statistics/RunningStatistics.cs

@@ -237,6 +237,15 @@ namespace MathNet.Numerics.Statistics
         /// </summary>
         public static RunningStatistics Combine(RunningStatistics a, RunningStatistics b)
         {
+            if (a._n == 0)
+            {
+                return b;
+            }
+            else if (b._n == 0)
+            {
+                return a;
+            }
+            
             long n = a._n + b._n;
             double d = b._m1 - a._m1;
             double d2 = d*d;