FIX: don't place null blocks on map

If the noteblock is neither a piano or percussion then make sure that no
null blocks are placed. This helps with future-proofing if a new music
type block is added but if Skylight doesn't implement it then it won't
be a very large issue.
Merge pull request #16 from Decagon/bugfix-note-block-null

FIX: don't place null blocks on map