When you use pick block on a sign, and place it again,
then all of the lines in the sign which are blank, are filled in
with the text "null". This also happens when you clone a sign
to another place, then the new sign will also have the bug.

Same happens for all of the signs in the whole world, when you rejoin the world.

I don't know if this matters, but all of the signs were placed
with /setblock.

In the attachments you can see a picture of the bug.
The lines with "null" were blank before this bug.