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 5dcbe701cf)

tools/fs_config/fs_config_generator.py

@@ -1133,6 +1133,8 @@ class FSConfigGen(BaseGenerator):
         path = fs_config.path
 
         if self._partition == 'system':
+            if not self._all_partitions:
+                return True
             for skip_partition in self._all_partitions.split(','):
                 if path.startswith(skip_partition) or path.startswith(
                         'system/' + skip_partition):