How to keep the spawnchunks loaded without generating the nether or end

What the OP requested is impossible to achieve.

I decompiled the minecraft 1.10 code using MCP and I can confirm what Skylinerw said. The only way to have entities processed in a dimension without players in it is by having an entity travel between that dimension and another dimension at least every 15 seconds.

If you are using modded version and can add a mod to the mod list, Mystcraft mod could be used: an island world, with no seas, will look very "empty" (like The End).

Aha! A video that shows a redstone contraption to keep chunks loaded.

Doesn't even need spawn chunks; the video demonstrates the difference between entity-active chunks and redstone-only active chunks.

UPDATE:

Ok, I've now watched a bunch of videos on chunkloading, and one on chunk unloading.

Turns out that chunk unloading is not "all-at-once", but rather "Unload 100 now, go back and unload more next tick".

Here is the key observation: If a chunk is loaded, and all 24 chunks in a 5x5 around it are also loaded, then that chunk will process both redstone and entities.

Have a 5x5 area? The 1x1 in the middle does both. Have a 7x7 area? The 3x3 in the middle does both. Etc.

For chunk unload protection, see this:

Basically, minecraft takes the X and Z chunk location, does an xor hash map, and that becomes the bucket it goes into. If it goes into a late enough bucket, and there are enough chunks loaded, the chunk will live long enough for hoppers to activate adjacent chunks, and force them to load, even if the game was trying to unload them.

So the trick becomes: 1. Have more than 100 chunks that want to be loaded, 2. Find out which chunks are "last" on that unload list hash order, 3. Have those chunks use hoppers to activate other chunks, which then reloads all your chunks. 4. Have a 2-chunk border around any chunk you want entity processing on.

In 1.14 and abobe this game mechanic had changed. You can`t load chuncks with reddstone or hoppers anymore. You have to fire items trough the nether or force an skeleton to shot arrows to a snow golem. You have to fire items every 15 seconds, if you do so you will charge a 5x5 area. Is better explained in this Ilmango's video: