fix: make task_context transforms really work without a task-context

It's not enough to make it optional in the schema, the actual code also
has to deal with its absence.

Thanks @ErichDonGubler!

Author: jcristau

src/taskgraph/transforms/task_context.py

@@ -85,7 +85,10 @@
 @transforms.add
 def render_task(config, tasks):
     for task in tasks:
-        sub_config = task.pop("task-context")
+        sub_config = task.pop("task-context", None)
+        if sub_config is None:
+            yield task
+            continue
         params_context = {}
         for var, path in sub_config.pop("from-parameters", {}).items():
             if isinstance(path, str):