Quote array index with -n so -n works properly

For some reason -n doesn't work with arrays, probably becuase
the array has an entry by the array's indexed value is empty.

By quoting it we get it as one unit, which behaves as expected.

Change-Id: Ic3243d13d3f8fc9978c460f816e33ad348302d8e

tools/build-all-docker-images

@@ -80,7 +80,7 @@ function init_image {
 
 function process_image {
     local image=$1
-    if [ -n ${dependency[$image]} ]; then
+    if [ -n "${dependency[$image]}" ]; then
         process_image ${dependency[$image]}
     fi
     if requires_build $image; then