posix: uname: 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 uname
version string:

```
*** Booting Zephyr OS build 3.5.0 ***

Printing everything in utsname...
sysname[7]: Zephyr
nodename[7]: zephyr
release[13]: 3.5.0
version[61]:  Apr  1 2024 23:48:30
machine[8]: riscv64

uart:~$
```

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 ***

Printing everything in utsname...
sysname[7]: Zephyr
nodename[7]: zephyr
release[13]: 3.5.0
version[61]: 3.5.0 Apr  1 2024 23:53:48
machine[8]: riscv64

uart:~$
```

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