dockerfile: ignore empty BUILDKIT_SYNTAX overrides

[crazy-max]

follow-up docker/buildx#3734

Treat an empty BUILDKIT_SYNTAX build argument as an explicit request to skip frontend forwarding. This lets builds fall back to the built-in Dockerfile frontend instead of trying to resolve an empty external frontend reference.

Before with docker/buildx#3734:

$ docker buildx build --build-arg BUILDKIT_SYNTAX="" https://github.com/docker/compose.git
#0 building with "default" instance using docker driver

#1 [internal] load git source https://github.com/docker/compose.git
#1 0.597 ref: refs/heads/main   HEAD
#1 0.597 b043368028e9fcb4545fa340c8ad635c370825da       HEAD
#1 1.046 b043368028e9fcb4545fa340c8ad635c370825da       refs/heads/main
#1 CACHED
ERROR: failed to build: failed to solve: failed with build-arg:BUILDKIT_SYNTAX = : invalid context name : invalid reference format

After with docker/buildx#3734:

$ docker buildx --builder builder build --build-arg BUILDKIT_SYNTAX="" https://github.com/docker/compose.git
#0 building with "builder" instance using docker-container driver

#1 [internal] load git source https://github.com/docker/compose.git
#1 0.598 ref: refs/heads/main   HEAD
#1 0.605 b043368028e9fcb4545fa340c8ad635c370825da       HEAD
#1 0.993 b043368028e9fcb4545fa340c8ad635c370825da       refs/heads/main
#1 0.059 Initialized empty Git repository in /var/lib/buildkit/runc-overlayfs/snapshots/snapshots/1/fs/
#1 0.644 ref: refs/heads/main   HEAD
#1 0.649 b043368028e9fcb4545fa340c8ad635c370825da       HEAD
#1 1.325 From https://github.com/docker/compose
#1 1.325  * [new branch]      main       -> main
#1 1.325  * [new branch]      main       -> origin/main
#1 1.329 b043368028e9fcb4545fa340c8ad635c370825da
#1 DONE 2.5s

#1 [internal] load git source https://github.com/docker/compose.git
#1 DONE 4.0s

#2 [internal] load metadata for docker.io/tonistiigi/xx:1.9.0
#2 ...