Make ninja targets optional.

This is to simplify build configurations removing empty properties.

Change-Id: Ic01f3852882d847a5997e98bb54085fb244ab450
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/41300
Commit-Queue: Godofredo Contreras <godofredoc@google.com>
Reviewed-by: Ricardo Amador <ricardoamador@google.com>

recipes/engine_v2/builder.py

diff --git a/recipes/engine_v2/builder.py b/recipes/engine_v2/builder.py
index 05096f0..f4ede89 100644
--- a/recipes/engine_v2/builder.py
+++ b/recipes/engine_v2/builder.py
@@ -156,7 +156,9 @@
     api.build_util.run_gn(build.get('gn'), checkout)
     ninja = build.get('ninja')
     ninja_tool[ninja.get('tool', 'ninja')](
-            ninja.get('config'), checkout, ninja.get('targets'))
+            ninja.get('config'),
+            checkout,
+            ninja.get('targets', []))
   generator_tasks = build.get('generators', {}).get('tasks', [])
   pub_dirs = build.get('generators', {}).get('pub_dirs', [])
   archives = build.get('archives', [])