fs_mgr: Do not mount empty partitions.

If a logical partition is resized to 0 bytes, it will have no extents.
This is not allowed by device-mapper, but is useful for effectively
compacting partitions with "fastboot flashall". If all logical
partitions are resized to 0, then resized to their intended size, then
we will allocate extents more efficiently.

However, if a partition is left with a zero size (either intentionally
or not), this should not throw the device into a reboot loop due to
CreateLogicalPartitions failing. Instead we skip partitions with no
extents.

Bug: 78793464
Test: with fastbootd:
        fastboot create-partition example 4096
        fastboot resize-partition example 0
      device reboots successfully

Change-Id: I572efa949176c8c3c493ef00438d8badd4d7cf4f