Reproduction steps:
1) Create new project
2) Create a new sprite (GameObject > 2D Object > Sprite)
3) Notice that from 5.4 Unity version it has a material in inspector ( or in "Sprite Renderer" component "Material" property is Sprites/Default)
4) Create a prefab of this sprite
6) Add prefab into a scene
5) Notice that added prefab's Material is missing (in "Sprite Renderer" component "Material" property is None (Material))

Expected result:
Material stays on Sprite after converting it into prefab.

Actual result:
After converting Sprite into prefab Material property is "None: instead of "Sprites/Default".

Note:
This happens only with Default material.

Workaround:
Create new material and apply it on sprite before converting it into a prefab.

Regression from 5.4.0b13