AP_Camera: image-start-capture with all zeros leads to infinite pics at zero interval

[rmackay9]

PR #24772 added support for the IMAGE_START_CAPTURE command's (time) Interval and Total Images fields.

While it interprets the command correctly, if all fields are left as zero the result is:

CameraId:0 = all cameras
Interval=0 (as quick as possible)
TotalImages=0 (capture forever)

So the camera library will take tons of images very quickly and also send tons of notifications to the GCS.

While it may not be exactly according to the mavlink spec I think we should interpret an all zero command as wanting to take just a single picture on all cameras immediately.

[rmackay9]

This branch should resolve this issue I think https://github.com/rmackay9/rmackay9-ardupilot/commits/camera-multipic-fix