Previously this was using `GPU_SHADER_TEXT` as default value indicating
an "unset" state. This wasn't documented in the definition (and so
D16284 added a new enumerator that broke this). Plus code was assuming
this enumerator would always have the value 0 without specifying this in
the definition either.
In this case it's easy to not rely on the enum value at all, and just
use `std::optional` to add a "unset" state.
Differential Revision: https://developer.blender.org/D16303
c81bc09876