Provide support for keystores with no/empty passphrase

[travis-bear]

I have several keystores with blank passphrases.

KeyStore.open() requires a passphrase param. Using None, \n, and '' don't work as valid values. There should be a way to open keystores that were created without a passphrase.

One solution would be to make the passphrase parameter optional.

[mahmoud]

Hey @travis-bear! You wouldn't happen to have one of those blank passphrase jks files to share would you? That way we could reproduce and add to our regression tests. I'd do it myself but keytool hasn't supported creating empty-passphrase JKS files for a while.