Config consumers should be able to recognize an empty config

For UX, it would be better if we presented an error for validation that
is "this config is empty" rather than the inaccurate "no server name" or
"no context name" errors. Returns a typed error.

pkg/client/unversioned/clientcmd/api/helpers.go

@@ -31,6 +31,14 @@ func init() {
 	redactedBytes = []byte(string(sDec))
 }
 
+// IsConfigEmpty returns true if the config is empty.
+func IsConfigEmpty(config *Config) bool {
+	return len(config.AuthInfos) == 0 && len(config.Clusters) == 0 && len(config.Contexts) == 0 &&
+		len(config.CurrentContext) == 0 &&
+		len(config.Preferences.Extensions) == 0 && !config.Preferences.Colors &&
+		len(config.Extensions) == 0
+}
+
 // MinifyConfig read the current context and uses that to keep only the relevant pieces of config
 // This is useful for making secrets based on kubeconfig files
 func MinifyConfig(config *Config) error {