build: ignore empty BUILDKIT_SYNTAX overrides

[crazy-max]

Do not switch to gateway.v0 when BUILDKIT_SYNTAX is present but empty. An empty value should not produce source="" and fail with invalid context name errors.

Before:

$ 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 0.896 ref: refs/heads/main   HEAD
#1 0.896 b043368028e9fcb4545fa340c8ad635c370825da       HEAD
#1 1.496 b043368028e9fcb4545fa340c8ad635c370825da       refs/heads/main
#1 0.051 Initialized empty Git repository in /var/lib/desktop-containerd/daemon/io.containerd.snapshotter.v1.overlayfs/snapshots/24841/fs/
#1 0.498 ref: refs/heads/main   HEAD
#1 0.498 b043368028e9fcb4545fa340c8ad635c370825da       HEAD
#1 1.353 From https://github.com/docker/compose
#1 1.353  * [new branch]      main       -> main
#1 1.354  * [new branch]      main       -> origin/main
#1 1.358 b043368028e9fcb4545fa340c8ad635c370825da
#1 DONE 3.1s
ERROR: failed to build: failed to solve: invalid context name : invalid reference format

After:

$ 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

This new error makes sense because BUILDKIT_SYNTAX="" still reaches BuildKit as build-arg:BUILDKIT_SYNTAX="" and treats it as an explicit syntax override, even when the value is empty. This should be fixed in BuildKit.