util-gem5art: Fix incorrect type of size in createArtifact

The typing here was `int` but had a default value of `None`. The correct
type is therefore `Optional[int]`.

Change-Id: Ibaf63151196b15f68e643fa5c1b290439d6618c8
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/66371
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>

util/gem5art/artifact/gem5art/artifact/artifact.py

@@ -158,7 +158,7 @@ class Artifact:
         documentation: str,
         inputs: List["Artifact"] = [],
         architecture: str = "",
-        size: int = None,
+        size: Optional[int] = None,
         is_zipped: bool = False,
         md5sum: str = "",
         url: str = "",