Fix 'show version' KeyError when sonic_version.yml has missing fields

'show version' crashes with KeyError when debian_version or
kernel_version are missing from sonic_version.yml. This happens in
docker-sonic-vs containers where the version file is generated without
these fields (they are only set during full image builds).

Use .get() with sensible runtime fallbacks:
- debian_version: 'N/A' (not available in container context)
- kernel_version: os.uname().release (actual host kernel at runtime)
- build_version: 'N/A'
- sonic_os_version: 'N/A'

Fixes sonic-net/sonic-buildimage#25765

Signed-off-by: securely1g <securely1g@users.noreply.github.com>

Author: securely1g

show/main.py

@@ -1695,10 +1695,10 @@ def version(verbose):
 
     sys_date = datetime.now()
 
-    click.echo("\nSONiC Software Version: SONiC.{}".format(version_info['build_version']))
-    click.echo("SONiC OS Version: {}".format(version_info['sonic_os_version']))
-    click.echo("Distribution: Debian {}".format(version_info['debian_version']))
-    click.echo("Kernel: {}".format(version_info['kernel_version']))
+    click.echo("\nSONiC Software Version: SONiC.{}".format(version_info.get('build_version', 'N/A')))
+    click.echo("SONiC OS Version: {}".format(version_info.get('sonic_os_version', 'N/A')))
+    click.echo("Distribution: Debian {}".format(version_info.get('debian_version', 'N/A')))
+    click.echo("Kernel: {}".format(version_info.get('kernel_version', os.uname().release)))
     click.echo("Build commit: {}".format(version_info['commit_id']))
     click.echo("Build date: {}".format(version_info['build_date']))
     click.echo("Built by: {}".format(version_info['built_by']))