Calls check-and-set-avb-args() when the system.img exists

Some targets, e.g., the newly added `gki_arm64`:
https://r.android.com/1935340, have no system.img.

To avoid the build error when making the vbmeta.img on a no-system-image
target, only sets system.img's AVB settings when it exists,.

Bug: 212486689
Test: `lunch gki_arm64-userdebug; make` with BOARD_AVB_ENABLE := true
Change-Id: I5e3c531e74116e421506a86ffce83848e7a9d6ea