Why is this not a valid Red-Black tree?
If you go to the empty leaf from the root in the pattern [Right, Left], you get to an empty leaf encountering 1 black node. If you go [Right, Right, Left] or [Right, Right, Right], you get to an empty leaf hitting 2 black nodes. This is not allowed in a Red Black Tree because by definition a path from root to an empty leaf should contain the same amount of black nodes as every other path from root to an empty leaf.

The other important thing to know is that NULL nodes (leaves) aren't drawn; only the internal nodes are drawn.  By convention, all leaves (NULL nodes) automatically are considered to be colored black.