/spreadplayers considers signs to be a valid block to be spread onto

The /spreadplayers command spreads people onto signs, making it possible to spawn above a high abyss.

Steps to reproduce

• Run the following command on a superflat with void preset

  /fill 10 -61 6 6 -61 10 minecraft:acacia_sign

• Run the spreadplayers command

  /spreadplayers 8 8 0 2 false @p

• Observe that you will spawn atop the signs and fall through the signs
• Replace some signs with solid blocks and observe that you will still be placed above the signs even though there are possible blocks to spread to.