Setblock signs with text put "null" on empty lines by reloading the world

Setblock signs with text put "null" in the empty boxes by reloading the world

When I restarted my world, all the empty boxes have been replaced by "null"
1 - Give a command block (/give @p command_block)
2 - Put this command and active it :

/setblock ~ ~ ~ minecraft:standing_sign 0 replace {Text1:"Hello !"}

3 - Look at your sign , it is normal
4 - Restart your world
5 - Look again the sign