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>
cd2c7874b9