Only pass cache if file is not empty

In commit 797862 the logic continues even if the cache file cannot be
downloaded. Unfortunately this approach is buggy since wget can create
empty files if the download fails.

To avoid passing an invalid cache file, check if file is not empty.

Signed-off-by: Walter Lozano <walter.lozano@collabora.com>

.gitlab-ci.yml

@@ -148,7 +148,7 @@ packaging-data-fetch-downstream:
         wget --header "JOB-TOKEN: $CI_JOB_TOKEN" "$ARTIFACT_URL" -O cache.yaml || true
       fi
     - |
-      if [ -e cache.yaml ] && [ "$DISABLE_CACHE" == "no" ]
+      if [ -s cache.yaml ] && [ "$DISABLE_CACHE" == "no" ]
       then
         echo Load cacheable data from cache.yaml 
         CACHE_ARGS="--cache cache.yaml"
@@ -192,7 +192,7 @@ packaging-data-fetch-sources-upstream:
         wget --header "JOB-TOKEN: $CI_JOB_TOKEN" "$ARTIFACT_URL" -O cache.yaml || true
       fi
     - |
-      if [ -e cache.yaml ] && [ "$DISABLE_CACHE" == "no" ] && [ "$FILTER_ON_CACHE" == "yes" ]
+      if [ -s cache.yaml ] && [ "$DISABLE_CACHE" == "no" ] && [ "$FILTER_ON_CACHE" == "yes" ]
       then
         echo Load cacheable data from cache.yaml
         CACHE_ARGS="--cache cache.yaml"