Improve error message when kubeconfig contains no clusters / contexts…

… / users.

When `kubectl config delete-context` and `kubectl config delete-cluster`
delete the last context / cluser / user, they set these fields to `null`
instead of empty lists. Before this change, these would cause deserialization
itself to fail because `null` cannot be deserialized into a `Vec`. It's better
that the test fails with the "couldn't find ..." error message instead.

k8s-openapi-tests/src/lib.rs

@@ -384,10 +384,13 @@ impl<'a, TResponseFuture, TResponse, R> Stream for MultipleValuesStream<'a, TRes
 
 #[derive(serde::Deserialize)]
 struct KubeConfig {
+    #[serde(deserialize_with = "deserialize_null_to_empty_vec")]
     clusters: Vec<KubeConfigClusterEntry>,
+    #[serde(deserialize_with = "deserialize_null_to_empty_vec")]
     contexts: Vec<KubeConfigContextEntry>,
     #[serde(rename = "current-context")]
     current_context: String,
+    #[serde(deserialize_with = "deserialize_null_to_empty_vec")]
     users: Vec<KubeConfigUserEntry>,
 }
 
@@ -492,6 +495,11 @@ mod methodstring {
     }
 }
 
+fn deserialize_null_to_empty_vec<'de, D, T>(deserializer: D) -> Result<Vec<T>, D::Error> where D: serde::Deserializer<'de>, T: serde::Deserialize<'de> {
+    let value: Option<Vec<T>> = serde::Deserialize::deserialize(deserializer)?;
+    Ok(value.unwrap_or_default())
+}
+
 mod api_versions;
 
 mod custom_resource_definition;