corrected bug when all variables are stationary and u is empty

(cherry picked from commit 9509a464)

matlab/get_variance_of_endogenous_variables.m

@@ -56,6 +56,7 @@ if size(u,2) > 0
     ghx = ghx(i_stat,:);
     ghu = ghu(i_stat,:);
 else
+    i_stat_0 = 1:size(ghx,2);
     i_stat = (1:n)';
 end