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.

crates/turborepo-lib/src/config.rs

@@ -299,11 +299,11 @@ impl ConfigurationOptions {
     }
 
     pub fn scm_base(&self) -> Option<&str> {
-        self.scm_base.as_deref()
+        non_empty_str(self.scm_base.as_deref())
     }
 
     pub fn scm_head(&self) -> &str {
-        self.scm_head.as_deref().unwrap_or("HEAD")
+        non_empty_str(self.scm_head.as_deref()).unwrap_or("HEAD")
     }
 
     pub fn allow_no_package_manager(&self) -> bool {
@@ -961,6 +961,8 @@ mod test {
         env.insert("turbo_daemon".into(), "".into());
         env.insert("turbo_env_mode".into(), "".into());
         env.insert("turbo_preflight".into(), "".into());
+        env.insert("turbo_scm_head".into(), "".into());
+        env.insert("turbo_scm_base".into(), "".into());
 
         let config = get_env_var_config(&env).unwrap();
         assert_eq!(config.api_url(), DEFAULT_API_URL);
@@ -972,6 +974,8 @@ mod test {
         assert_eq!(config.daemon, None);
         assert_eq!(config.env_mode, None);
         assert!(!config.preflight());
+        assert_eq!(config.scm_base(), None);
+        assert_eq!(config.scm_head(), "HEAD");
     }
 
     #[test]

crates/turborepo-lib/src/run/scope/change_detector.rs

@@ -93,26 +93,23 @@ impl<'a> GitChangeDetector for ScopeChangeDetector<'a> {
         include_uncommitted: bool,
         allow_unknown_objects: bool,
     ) -> Result<HashSet<PackageName>, ResolutionError> {
-        let mut changed_files = HashSet::new();
-        if !from_ref.map_or(false, |s| s.is_empty()) {
-            changed_files = match self.scm.changed_files(
-                self.turbo_root,
-                from_ref,
-                to_ref,
-                include_uncommitted,
-                allow_unknown_objects,
-            )? {
-                ChangedFiles::All => {
-                    debug!("all packages changed");
-                    return Ok(self
-                        .pkg_graph
-                        .packages()
-                        .map(|(name, _)| name.to_owned())
-                        .collect());
-                }
-                ChangedFiles::Some(changed_files) => changed_files,
+        let changed_files = match self.scm.changed_files(
+            self.turbo_root,
+            from_ref,
+            to_ref,
+            include_uncommitted,
+            allow_unknown_objects,
+        )? {
+            ChangedFiles::All => {
+                debug!("all packages changed");
+                return Ok(self
+                    .pkg_graph
+                    .packages()
+                    .map(|(name, _)| name.to_owned())
+                    .collect());
             }
-        }
+            ChangedFiles::Some(changed_files) => changed_files,
+        };
 
         let lockfile_contents = self.get_lockfile_contents(from_ref, &changed_files);