kernel: banner: use BUILD_VERSION only if not empty

The `BUILD_VERSION` can be defined but empty when built
without git, causing version to be missing from the banner:

```
*** Booting Zephyr OS build  ***
Hello World! qemu_riscv64
```

Let's check if it is empty before using it, so that
`KERNEL_VERSION_STRING`, which is generated independently
with cmake can be used as a fallback:

```
*** Booting Zephyr OS build 3.5.0 ***
Hello World! qemu_riscv64
```

Signed-off-by: Yong Cong Sin <ycsin@meta.com>