Fix NoMethodError: undefined method []' for nil:NilClass`

`@response` can be `nil`.

lib/kubeclient/http_error.rb

@@ -15,7 +15,7 @@ def initialize(error_code, message, response) # rubocop:disable Lint/MissingSupe
 
   def to_s
     string = "HTTP status code #{@error_code}, #{@message}"
-    if @response[:request]
+    if @response && @response[:request]
       request_method = @response[:request][:method]&.to_s&.upcase
       request_path = @response[:request][:url_path]
       string += " for #{request_method} #{request_path}"