runner.aws_batch: Omit vcpus and memory from the job submission when …

…unspecified

These parameters may be None, which is not allowed in the job
submission.  Trying to do so raises an exception.

nextstrain/cli/runner/aws_batch/jobs.py

@@ -142,8 +142,8 @@ def submit(name: str,
                 },
                 *forwarded_environment(),
             ],
-            "vcpus": cpus,
-            "memory": memory,
+            **({ "vcpus": cpus } if cpus else {}),
+            **({ "memory": memory } if memory else {}),
             "command": [
                 "/sbin/entrypoint-aws-batch",
                 *exec