The Normal vector socket in the Normal node wasn't drawn properly and
couldn't be controlled. Additionally, the socket name was drawn over it.
This happened because the socket had a default value of a zero vector.
To fix this, we set the default value to the unit vector `(0, 0, 1)`.
Moreover, we don't draw the UI name if the subtype is `PROP_DIRECTION`.
Reviewers: brecht
Differential Revision: https://developer.blender.org/D5503
25f1783673