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

scripts/conv_linker_config.py

@@ -28,6 +28,7 @@ from google.protobuf.text_format import MessageToString
 
 def Proto(args):
     pb = linker_config_pb2.LinkerConfig()
+    if args.source:
         for input in args.source.split(':'):
             json_content = ''
             with open(input) as f:
@@ -104,7 +105,7 @@ def GetArgParser():
     parser_proto.add_argument(
         '-s',
         '--source',
-        required=True,
+        nargs='?',
         type=str,
         help='Colon-separated list of linker configuration files in JSON.')
     parser_proto.add_argument(