Support nvlPrev,nvlNext window function

[navis]

by request.

    testQuery(
        "WITH X AS ("
        + " SELECT * FROM "
        + " (VALUES"
        + "   (0, 'A', 1, null, null),"
        + "   (0, 'B', null, 2, null),"
        + "   (0, 'C', null, null, 3),"
        + "   (1, 'A', 4, null, null),"
        + "   (1, 'B', null, 5, null),"
        + "   (1, 'C', null, null, 6)"
        + " ) AS T (__time, asset, p1, p2, p3)"
        + ")"
        + " SELECT nvlPrev(p1) over (), nvlNext(p1) over (),"
        + "        nvlPrev(p2) over (), nvlNext(p2) over (),"
        + "        nvlPrev(p3) over (), nvlNext(p3) over () FROM X",
        new Object[]{1L, 1L, null, 2L, null, 3L},
        new Object[]{1L, 4L, 2L, 2L, null, 3L},
        new Object[]{1L, 4L, 2L, 5L, 3L, 3L},
        new Object[]{4L, 4L, 2L, 5L, 3L, 6L},
        new Object[]{4L, null, 5L, 5L, 3L, 6L},
        new Object[]{4L, null, 5L, null, 6L, 6L}
    );