fs_config: Fix cases without vendor/oem partition

If the device has neither an OEM nor a vendor partiton,
$PRIVATE_PARTITION_LIST will be empty, causing the fsconfig tool to fail
because the "--all-partitions" argument will be empty.

Quote the comma-substituted "$(PRIVATE_PARTITION_LIST)" - which may be
empty - to appease argparse, which will then populate "_all_partitions"
with an empty "str" object.

Checking _all_partitions against None is superfluous since either
argparse will catch the empty argument and fail early, or the argument
will be a string, in which case it will not be None.

Bug: 140578676
Test: build only a system partition
Signed-off-by: Felix <google@ix5.org>
Change-Id: I236e30445b303b4945467b5dc4387d54b5d984f9
(cherry picked from commit 8453f02c)