Adjust get_sources.sh to allow empty sources file

There are situations in which one will want to use a `sources` file to
indicate that they wish to use the flat dist-git layout.
However, until now we did not allow empty `sources` file in flat dist-git
layout.
With this change we allow empty `sources` file and we will just echo
something in the logs saying that this file is empty and bail.

Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>

get_sources.sh

@@ -146,7 +146,10 @@ else
   done <<< "$(git branch -r --contains HEAD | sed 's#origin/##g')"
 fi
 
-if [[ -s sources ]]; then
+if [[ -f sources ]]; then
+    if [[ ! -s sources ]]; then
+      echo "Empty sources file -- nothing to check"
+    else
       # This section is for the "flat" dist-git layout, where the spec file and
       # patches are all present at the top level directory and the sha of the tarball
       # present in a 'sources' file.
@@ -192,6 +195,7 @@ if [[ -s sources ]]; then
           fi
       done < sources
       "${hashtype}sum" -c sources
+    fi
 else
     # This section is for the "non-flat" dist-git layout, where the spec file
     # is stored in a SPECS folder, the patches in a SOURCES folder and the sha