Fix nil panic in UpdateCache when listapisresponse is empty (#211)

[rohanmishra29]

Fixes #211

Problem

When the CloudStack management server returns an empty listapisresponse (i.e., {"listapisresponse":{}}), the sync command panics with:
panic: interface conversion: interface {} is nil, not []interface {}
This happens in config/cache.go at the type assertion response["api"].([]interface{}) when response["api"] is nil.

Fix

Added a nil check for response["api"] before the type assertion. If the API list is empty, the function now returns nil with a clear error message instead of panicking.

Testing

Built successfully with go build ./.... The nil check prevents the panic when an empty listapisresponse is received.