build: non-tag images self-report an empty version (option_env! sees Some(""))

Observed while validating #967 (pre-existing, not introduced there).

Dockerfile declares ARG BUILD_VERSION= and materializes it as ENV AISIX_BUILD_VERSION=$BUILD_VERSION unconditionally, so every non-tag build (main :dev, :poc, workflow_dispatch, plain local docker build) compiles with AISIX_BUILD_VERSION set to the EMPTY STRING. crates/aisix-core/src/version.rs:14 uses option_env!, which returns Some("") for a set-but-empty variable, so the documented fallback to CARGO_PKG_VERSION never triggers:

docker run --rm --entrypoint /usr/local/bin/aisix ghcr.io/api7/aisix:dev --version
aisix

(bare aisix, no version — likewise the Server header and heartbeat dp_version on those builds).

Tag builds pass a real BUILD_VERSION and are unaffected; the tag-time version check only runs on tags, so this never trips CI.

Fix direction: treat empty as unset, e.g. a match arm Some(v) if !v.is_empty() (const-compatible via manual byte-len check) or guard the ENV in the Dockerfile. The unit test at version.rs:24 asserts non-empty but only runs against the crate-version fallback in CI, which is why it never caught this.

Start with Dockerfile and crates/aisix-core/src/version.rs:14, then read the unit test at version.rs:24. Reproduce a non-tag build where AISIX_BUILD_VERSION is empty and verify that the crate-version fallback is used. Done means --version, the Server header, and heartbeat dp_version report a non-empty version for non-tag builds while tag builds remain unaffected.