Change SuperImageProperties.Block_devices to be a slice of string

There are some products have empty BOARD_SUPER_PARTITION_BLOCK_DEVICES,
which is valid. But the previous super image logic didn't handle
it correctly. This CL fixes the issue.

Test: CI and verified on the product that has PRODUCT_BUILD_SUPER_PARTITION
      true and empty BOARD_SUPER_PARTITION_BLOCK_DEVICES
Change-Id: I761848ce6abc3f75a5b2083b79008f236e08b4fb