Empty lines on old signs show null
If a sign was placed in an older version with one or more empty lines, and is then loaded in the snapshot, these lines will display the text null. Affects signs placed in 1.7.x versions, and signs placed in 1.8.8 are not affected. Unsure about older 1.8.x versions. These are likely the same signs that caused MC-82968.
Linked Issues
Created Issue:
Empty lines on old signs show null
If a sign was placed in an older version with one or more empty lines, and is then loaded in the snapshot, these lines will display the text null. Affects signs placed in 1.7.x versions, and signs placed in 1.8.8 are not affected. Unsure about older 1.8.x versions. These are likely the same signs that caused
MC-82968.
Added Linked Issues:
duplicates

Did you open the world in any snapshot earlier than 15w32b ?
Did you open the world in any earlier version after opening in 15w32b ?
First question: if by any snapshot you mean any 1.9 snapshot, then no. However, the world has previously been loaded in snapshots for older versions.
Second question: no.
Did you open the world in 15w31a?
As previously stated, no.
This ticket has been marked as a duplicate of
MC-85627, in favor of the more general or better information provided.