[FIX] OUI availability check implementation

[Krzysztofz01]

Minor change to the OUI class, which now includes an additional boolean value to indicate if Properties initialization was successful. Although Properties is initialized, it is partially protected against "Partially initialized instances". Moreover, I added a method that can check if OUI data is available. When Properties does not contain a given key, the return value is null, but keeping the information about data availability we do not have to rely on this mechanic.

[bobzilladev]

The data returned will be null if the property isn't found, so this is logically equivalent to the existing code, so going to close this one.