The default value of any type T is default(T), so for 100% theoretical accuracy you would write
if (!mapping.Equals(default(KeyValuePair<string, int>))) {
// ...
}
Since KeyValuePair is a struct (i.e. a value type) you cannot compare it to null. Irrespective of that, comparing values with == is wrong as a default approach because if used on reference types it normally checks for reference equality.