fs_config: Fix generation for devices without vendor partition

If the "--all-partitions" argument is empty, the system fs_config should
be generated with all partitions emitted.

Bug: 140578676
Test: build with only a system partition and check fs_config result
Fixes: 8453f02c (fs_config: Fix cases without vendor/oem partition)
Change-Id: I058cfd1c8e665b40757998e773c35e26465bf58a
Signed-off-by: dianlujitao <dianlujitao@lineageos.org>
(cherry picked from commit 5dcbe701)