Issue using defaultClient on Windows

[falken]

Running this code on windows
ApiClient client = Config.defaultClient();

Generates this exception:
java.lang.NullPointerException
at java.io.File.(File.java:277)
at io.kubernetes.client.util.ClientBuilder.findHomeDir(ClientBuilder.java:121)
at io.kubernetes.client.util.ClientBuilder.findConfigInHomeDir(ClientBuilder.java:149)
at io.kubernetes.client.util.ClientBuilder.standard(ClientBuilder.java:89)
at io.kubernetes.client.util.ClientBuilder.standard(ClientBuilder.java:75)
at io.kubernetes.client.util.Config.defaultClient(Config.java:104)

The problem seems to be that System.getenv(ENV_HOME) in this line:
final File config = new File(System.getenv(ENV_HOME));
returns null, causing the File constructor to fail.

I would suggest a simple null check or default to empty string.

[brendandburns]

This was fixed at HEAD by #455

Looks like I need to cut a new release....

[brendandburns]

Release 4.0.0 was cut in february so this should be fixed there.