stats: fix Vector2d to display stats correctly when y_subname is not specified.
stats: fix Vector2d to display stats correctly when y_subname is not specified.

Vector2d stats with no y_subname were not displayed as the VectorPrint subname was not initialized correctly to reflect the empty field.

haowu4682/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5,haowu4682/gem5,haowu4682/gem5,haowu4682/gem5,LingxiaoJIA/gem5,haowu4682/gem5,haowu4682/gem5,haowu4682/gem5,LingxiaoJIA/gem5,haowu4682/gem5,LingxiaoJIA/gem5,haowu4682/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5