The reason your 1.5 world becomes void is because 1.8 does not have a converter into the new name-based format. It only can read them, not convert.
Note: If you have already opened your worlds in 1.8, you CANNOT retrieve it unless you have a backup.
Firstly, see if you can open them in 1.5 (the version you last used).
If that works, try opening it in 1.7. Save and quit after it loads.
Now, your world is ready to be open in 1.8, as 1.7 has automatically prepared your world for the new name-based format (when you loaded it).
One more thing: It may be wise to turn on cheats on your world by changing
allowCommands:1 in your level.dat file (so that you can fly and not continuously fall into the void as your spawn point will most likely be void as it was in a generated chunk that 1.8 converted into air due to unknown values; numbers instead of names)