Disable sparse only when partition_size is undefined.

Currently, filesystem module automatically adds `disable_sparse` entry
when generating the prop file. However, this property should not be
added when partition_size is specified, given that partition_size
property is introduced in filesystem module with
https://r.android.com/3451348.

Test: m out/soong/.intermediates/build/soong/fsgen/aosp_cf_x86_64_phone_generated_userdata_image/android_common/userdata.img
Bug: 388920173
Change-Id: Ia86210be77284c71f3831464149789b934c55001