gn2bp: check if file passed in extras exists

We initially have no need to use an Android.bp.extras file.

Test: n/a
Change-Id: I97d16d59918fd6d59c7007d75603822167f3b496
diff --git a/tools/gn2bp/gen_android_bp b/tools/gn2bp/gen_android_bp
index 127d27d..ed4a412 100755
--- a/tools/gn2bp/gen_android_bp
+++ b/tools/gn2bp/gen_android_bp
@@ -1099,9 +1099,10 @@
 """ % (tool_name)
   ]
   blueprint.to_string(output)
-  with open(args.extras, 'r') as r:
-    for line in r:
-      output.append(line.rstrip("\n\r"))
+  if os.path.exists(args.extras):
+    with open(args.extras, 'r') as r:
+      for line in r:
+        output.append(line.rstrip("\n\r"))
 
   out_files = []