Allow specifying null vbmeta.img for production system.img

For a system image that is not a GSI but built for a specific
product (e.g. Pixel 2 system image), in order to be
flashed on a device of an older Android version, the vbmeta.img
must be replaced by a null vbmeta.img built by avbtool.

Bug: 78215452
Test: development/vndk/tools/build_mixed -v 8.1.0
-m development/vndk/tools/modify_system_img.sh
-p <null_vbmeta_image_path> gsi_build_dir device_build_dir out_dir
<check_tool>

Change-Id: I5bc9f541f177205d548a0e2e97eb169686567bd5