fix(affected): don't respect empty scm env vars (#9053)
### Description
Users can set empty string env vars which will end up getting chosen
over our defaults. Empty env vars can accidentally get set when using
[`docker
compose`](https://docs.docker.com/compose/environment-variables/set-environment-variables/#additional-information)
if a pass through env is declared in the YAML, but the shell does not
have the var defined.
This means that `Some("")` will no longer be a valid value in for
`from_ref`. This is already the case for `TargetSelector`s from
`--filter`s:
[source](https://github.com/vercel/turborepo/blob/main/crates/turborepo-lib/src/run/scope/target_selector.rs#L145)
This also allows us to remove the awkward comparison where we check if
the scm base and skip calculating changes if it is `Some("")`, but do
the calculation if it is `None`.
### Testing Instructions
Added unit test to verify that empty scm env vars will not be respected.
