runtime-rs: Fix issues for empty initdata annotation test

Currently, there are 2 issues for the empty initdata annotation
test:

- Empty string handling
- "\[CDH\] \[ERROR\]: Get Resource failed" not appearing

`add_hypervisor_initdata_overrides()` does not handle
an empty string, which might lead to panic like:

```
called `Result::unwrap()` on an `Err` value: gz decoder failed
Caused by:
    failed to fill whole buffer
```

This commit makes the function return an empty string
for a given empty input and updates the assertion string
to one that appears in both go-runtime and runtime-rs.

Signed-off-by: Alex Lyn <alex.lyn@antgroup.com>
Signed-off-by: Hyounggyu Choi <Hyounggyu.Choi@ibm.com>

tests/integration/kubernetes/k8s-initdata.bats

@@ -182,7 +182,7 @@ EOF
         return 1
     fi
 
-    assert_logs_contain "${node}" kata "${node_start_time}" "\[CDH\] \[ERROR\]: Get Resource failed"
+    assert_logs_contain "${node}" kata "${node_start_time}" "\[CDH\] \[ERROR\]: Image Client error: Initialize resource provider failed: Get resource failed"
 }
 
 teardown() {