docker buildx no longer likes blank strings

Author: amarburg

.github/workflows/docker.yaml

@@ -130,4 +130,4 @@ jobs:
           set: |
             *.platform=linux/amd64
             *.cache-from=type=registry,ref=ghcr.io/${{ steps.lowercase-repo.outputs.repository }}:cache-${{ matrix.ROS_DISTRO }}
-            *.cache-to=
+            *.cache-to=''