Add AllowMissingDependencies support for prebuilt_etc module with no src property

Arch-specific prebuilt_etc modules may be missing source files for new
architectures.  Allow build analysis to continue when there is no
source file when AllowMissingDependencies is set.

Bug: 250918230
Test: lunch aosp_riscv64-userdebug && m ALLOW_MISSING_DEPENDENCIES=true nothing
Test: TestPrebuiltEtcAllowMissingDependencies
Change-Id: I647c7305339e3ed80283be5e59e6f4ef15ae2384