Fix K8S_OPTIONS maybe empty issue (sonic-net#16891)

Why I did it
K8S_OPTIONS maybe empty, so there will be syntax error. Need to fix this issue.

Work item tracking
Microsoft ADO (number only): 25495020
How I did it
Add "" for K8S_OPTIONS to avoid exception.

How to verify it
No more exception is throwed in PR build validation pipeline.

.azure-pipelines/azure-pipelines-build.yml

@@ -128,7 +128,7 @@ jobs:
               make $BUILD_OPTIONS ENABLE_ASAN=y target/docker-sonic-vs.gz
               mv target/docker-sonic-vs.gz target/docker-sonic-vs-asan.gz
             fi
-            if [ $(K8S_OPTIONS) == 'INCLUDE_KUBERNETES_MASTER=y' ]; then
+            if [ "$(K8S_OPTIONS)" == 'INCLUDE_KUBERNETES_MASTER=y' ]; then
               make $BUILD_OPTIONS $(K8S_OPTIONS) target/sonic-vs.img.gz
               mv target/sonic-vs.img.gz target/sonic-vs-k8s.img.gz
             fi