Fix for TimestampFlag.GetValue to return empty string without value

flag_timestamp.go

@@ -75,7 +75,7 @@ func (t *Timestamp) Get() interface{} {
 // GetValue returns the flags value as string representation and an empty
 // string if the flag takes no value at all.
 func (f *TimestampFlag) GetValue() string {
-	if f.Value != nil {
+	if f.Value != nil && f.Value.timestamp != nil {
 		return f.Value.timestamp.String()
 	}
 	return ""