conv_linker_config proto works with empty input

PRODUCT_VENDOR_LINKER_CONFIG_FRAGMENTS lists json files for vendor
linker config. It's annoying to handle the case of empty list.

`proto` subcommand now works for empty input. This is useful to generate
the empty linker config.

Bug: 244531518
Test: conv_linker_config proto --source --output output.pb
Change-Id: Iec6de67a979814a818730c393d9a4a7ca5d2eebe