Returns the first available principal from any of the {@code Realm} principals, or {@code null} if there are
no principals yet.
<p/>
The 'first available principal' is interpreted as the principal that would be returned by
<code>{@link #iterator() iterator()}.{@link java.util.Iterator#next() next()}.</code>
Returns the first available principal from any of the {@code Realm} principals, or {@code null} if there are no principals yet. <p/> The 'first available principal' is interpreted as the principal that would be returned by <code>{@link #iterator() iterator()}.{@link java.util.Iterator#next() next()}.</code>
@inheritDoc