camera aidl: 'empty' native handles not 'null' native handles

parcelable StreamBuffer {

    /**

     * The ID of the stream this buffer is associated with. -1 indicates an

     * invalid (empty) StreamBuffer, in which case buffer must also point to

     * null and bufferId must be 0.

     * invalid (empty) StreamBuffer, in which case buffer must be empty

     * and bufferId must be 0.

     */

    int streamId;

     * corresponding stream is removed from stream configuration or until camera

     * device session is closed. After the first time a buffer is introduced to

     * HAL, in the future camera service must refer to the same buffer using

     * only bufferId, and keep the buffer handle null.

     * only bufferId, and keep the buffer handle empty.

     */

    long bufferId;

     * is not seen by the HAL before, this buffer handle is guaranteed to be a

     * valid handle to a graphics buffer, with dimensions and format matching

     * that of the stream. If the bufferId has been sent to the HAL before, this

     * buffer handle must be null and HAL must look up the actual buffer handle

     * buffer handle must be empty and HAL must look up the actual buffer handle

     * to use from its own bufferId to buffer handle map.

     *

     * For StreamBuffers returned in a CaptureResult, this must be null, since

     * For StreamBuffers returned in a CaptureResult, this must be empty, since

     * the handle to the buffer is already known to the client (since the client

     * sent it in the matching CaptureRequest), and the handle can be identified

     * by the combination of frame number and stream ID.

     * The acquire sync fence for this buffer. The HAL must wait on this fence

     * fd before attempting to read from or write to this buffer.

     *

     * In a buffer included in a CaptureRequest, the client may set this to null

     * In a buffer included in a CaptureRequest, the client may leave this empty

     * to indicate that no waiting is necessary for this buffer.

     *

     * When the HAL returns an input or output buffer to the framework with

     * processCaptureResult(), the acquireFence must be set to null. If the HAL

     * processCaptureResult(), the acquireFence must be empty. If the HAL

     * never waits on the acquireFence due to an error in filling or reading a

     * buffer, when calling processCaptureResult() the HAL must set the

     * releaseFence of the buffer to be the acquireFence passed to it by the

    /**

     * The release sync fence for this buffer. The HAL must set this to a valid

     * fence fd when returning the input buffer or output buffers to the client

     * in a CaptureResult, or set it to null to indicate that no waiting is

     * in a CaptureResult, or leave it empty to indicate that no waiting is

     * required for this buffer.

     *

     * The client must set this to be null for all buffers included in a

     * The client must leave this empty for all buffers included in a

     * processCaptureRequest call.

     *

     * After signaling the releaseFence for this buffer, the HAL

     * must not make any further attempts to access this buffer as the

     * ownership has been fully transferred back to the client.

     *

     * If this is null, then the ownership of this buffer is transferred back

     * If this is empty, then the ownership of the buffer is transferred back

     * immediately upon the call of processCaptureResult.

     */

    NativeHandle releaseFence;