[build] When generating image version, handle case where current comm…

…it has no reachable tags (#2506)

Signed-off-by: Joe LeVeque <jolevequ@microsoft.com>

functions.sh

@@ -60,7 +60,7 @@ sonic_get_version() {
     BUILD_NUMBER=${BUILD_NUMBER:-0}
     ## Check if we are on tagged commit
     ## Note: escape the version string by sed: / -> _
-    if [ "$describe" == "$latest_tag" ]; then
+    if [ -n "$latest_tag" ] && [ "$describe" == "$latest_tag" ]; then
         echo "${latest_tag}${dirty}" | sed 's/\//_/g'
     else
         echo "${branch_name}.${BUILD_NUMBER}${dirty:--$(git rev-parse --short HEAD)}" | sed 's/\//_/g'