acpid: handle getdents on empty Thermal and Power dirs

thermald and redbear-upower read_dir /scheme/acpi/{thermal,power} to
enumerate ACPI _TZ zones and _PR power sources. The acpid scheme
returned EIO for these new directory variants, which std::fs::read_dir
interprets as 'the path is not a directory or doesn't exist' and
emits a warning.

Return Ok with no entries for Thermal/Power getdents so read_dir
sees an existing-but-empty directory and consumers gracefully fall
through to the empty-state path.

drivers/acpid/src/scheme.rs

@@ -407,6 +407,13 @@ impl SchemeSync for AcpiScheme<'_, '_> {
                     })?;
                 }
             }
+            HandleKind::Thermal | HandleKind::Power => {
+                // Empty placeholder directories. Consumers (thermald,
+                // redbear-upower) iterate them with read_dir and gracefully
+                // handle the empty result. Returning Ok with no entries
+                // is what `read_dir` expects for an existing-but-empty
+                // directory.
+            }
             _ => return Err(Error::new(EIO)),
         }