agent: fix the issue of setup sandbox pidns

When the sandbox api was enabled, the pasue container
wouldn't be created, thus the shared sandbox pidns
should be fallbacked to the first container's init process,
instead of return any error here.

Signed-off-by: Fupan Li <fupan.lfp@antgroup.com>

src/agent/src/rpc.rs

@@ -1750,7 +1750,7 @@ fn update_container_namespaces(
                 if !pidns.path.is_empty() {
                     pid_ns.set_path(Some(PathBuf::from(&pidns.path)));
                 }
-            } else {
+            } else if !sandbox.containers.is_empty() {
                 return Err(anyhow!(ERR_NO_SANDBOX_PIDNS));
             }
         }
@@ -2527,14 +2527,6 @@ mod tests {
                     .unwrap()],
                 ..Default::default()
             },
-            TestData {
-                namespaces: vec![],
-                sandbox_pidns_path: None,
-                use_sandbox_pidns: true,
-                result: Err(anyhow!(ERR_NO_SANDBOX_PIDNS)),
-                expected_namespaces: vec![],
-                ..Default::default()
-            },
             TestData {
                 has_linux_in_spec: false,
                 result: Err(anyhow!(ERR_NO_LINUX_FIELD)),