Fix node shell secret pulling (#3668)

- On some KE errors are thrown if trying to get a secret with an empty
  name, instead of just ignoring the entry

- Clean up the node shell settings

Signed-off-by: Sebastian Malton <sebastian@malton.name>

src/main/cluster.ts

@@ -706,10 +706,10 @@ export class Cluster implements ClusterModel, ClusterState {
   }
 
   get nodeShellImage(): string {
-    return this.preferences.nodeShellImage || initialNodeShellImage;
+    return this.preferences?.nodeShellImage || initialNodeShellImage;
   }
 
-  get imagePullSecret(): string {
-    return this.preferences.imagePullSecret || "";
+  get imagePullSecret(): string | undefined {
+    return this.preferences?.imagePullSecret;
   }
 }