Previously, the input socket drawing code did not cover all cases with required
and optional labels for all socket types. E.g. menu sockets were never drawn
with label (unless there was some error) regardless of whether `optional_label`
was set or not.
This patch unifies the handling of this flag, i.e. all socket types support
drawing with and without label. This is also required before we can expose this
functionality to users.
Since all menu inputs had optional labels by default before, this patch makes
labels optional explicitly by changing the declaration of all the nodes. This
way, there is no expected user-visible change with this patch.
Pull Request: https://projects.blender.org/blender/blender/pulls/146935
f79896f5b9