set default passphrase to nil instead of the empty string.

[mpvosseller]

This fixes two bugs in the apn command line tool.

• APN_CERTIFICATE_PASSPHRASE was never being used because the passphrase would always have a value (the one provided by the user or the empty string).
• When using an unencrypted pem (not password protected) the command line would complain unless you provided some 4+ byte value password. This was because when no password was provided it would use the empty string and complain that the password was not long enough. See [comment] (Exception sending notification: password must be at least 4 bytes #146 (comment))

This pull request allows the password to be nil to fix the above issues.

[dankimio]

@mpvosseller thank you!