fix(cri): reject empty image reference in pull/status/remove image

PullImage, ImageStatus and RemoveImage validated that the image SPEC was present
but not that image_spec.image was non-empty. An empty reference passed the check
and failed deep in reference parsing with a confusing error. Reject it up front
with a clear invalid_argument, consistently across the three image RPCs.

From the CRI audit (hand-verified). clippy -D warnings clean; 237 cri tests pass.

Author: Roy Lin

src/cri/src/image_service.rs

@@ -225,6 +225,13 @@ impl ImageService for BoxImageService {
         let image_spec = req
             .image
             .ok_or_else(|| Status::invalid_argument("image spec required"))?;
+        if image_spec.image.trim().is_empty() {
+            // Reject an empty reference up front; otherwise it fails deep in
+            // reference parsing with a confusing error.
+            return Err(Status::invalid_argument(
+                "image reference must not be empty",
+            ));
+        }
 
         let digest = self.resolve_digest(&image_spec.image).await;
 
@@ -258,6 +265,13 @@ impl ImageService for BoxImageService {
         let image_spec = req
             .image
             .ok_or_else(|| Status::invalid_argument("image spec required"))?;
+        if image_spec.image.trim().is_empty() {
+            // Reject an empty reference up front; otherwise it fails deep in
+            // reference parsing with a confusing error.
+            return Err(Status::invalid_argument(
+                "image reference must not be empty",
+            ));
+        }
 
         // Honor kubelet's per-request credentials (imagePullSecrets). When the
         // request carries usable auth, pull with a one-off puller built from it;
@@ -298,6 +312,13 @@ impl ImageService for BoxImageService {
         let image_spec = req
             .image
             .ok_or_else(|| Status::invalid_argument("image spec required"))?;
+        if image_spec.image.trim().is_empty() {
+            // Reject an empty reference up front; otherwise it fails deep in
+            // reference parsing with a confusing error.
+            return Err(Status::invalid_argument(
+                "image reference must not be empty",
+            ));
+        }
 
         tracing::info!(image = %image_spec.image, "CRI RemoveImage");