ImpersonatedCredentials can request 0s lifetime

[chingor13]

When going through the builder, it's possible for the lifetime value to be unset (defaulting to 0 in Java).

ImpersonatedCredentials credentials = ImpersonatedCredentials.newBuilder()
    .setScopes(ImmutableList.of("some scope"))
    .build();
// This builds, but has a lifetime of `0`

When we go to refresh the token, we don't check for 0. It's unclear what sending 0s for the lifetime value will do on the backend. In the above case where you don't specify a lifetime, it seems that you would be requesting the default (whether client specified or backend specified).