Since porting this to C in rB850234c1b10a, Smart UV Project would not
add a UVMap (if none existed already) anymore.
Not having a UVMap already is a reasonable situation though when e.g.
starting off fresh by deleting an existing UVMap or also when applying
certain generative modifiers. This is also inconsistent with all other
unwrap operators (all of them will create a UVMap if none exists
already)
Differential Revision: https://developer.blender.org/D9001
748efc710c