Null maps have a few variants, so each of the variants will be described differently on this page.
Variant 1: Traditional null maps
Before a patch was released, creating a map in the hub and then throwing it on the ground (without holding it in your hand) would make it become a "null" map.
The value of these nulls varies from 1 to any number higher, although it was limited to how much inventory space the player had.
Tons of null maps could easily be created in a matter of minutes, so this variant isn't that rare.
Variant 2: Toggle maps
Author's note: I do not know much about the obtaining of these. This section may inaccurate.
Toggle maps are a unique form of null maps. There are a few features that set the "toggle map" apart from the
First, they have a damage value of 0. No "normal" null maps have been identified to have a value of 0.
Secondly, they change from a "null" map to a regular map if you change servers with it in your inventory. Holding the map in your hand will change it back int a "null" map. Hence the name, "toggle," map.
Thirdly and finally, they are not dupe-able. There is a vanilla minecraft feature where you can put a filled map and an empty map in the crafting menu to create two of the original map, duplicating it. Original null maps are dupeable, but since the patch for null maps was released, the new, present day non-null maps are not dupe-able.