Decode partitioins string stored outside.
Example
iex> decode_partitions(nil)
[]
iex> decode_partitions("")
[]
iex> decode_partitions("[1,2,3,4,5]")
[1,2,3,4,5]
iex> decode_partitions("[1,2,3,]")
[1,2,3]
iex> decode_partitions("[1,2,3")
[1,2,3]
iex> decode_partitions("1,2,3")
[1,2,3]