nixos/pleroma: regenerate empty release cookie files

Since b9cfbcafdf, the lack of hexdump in
the closure lead to the generation of empty cookie files. This empty
cookie file is making pleroma to crash at startup now we correctly
read it.

We introduce a migration forcing these empty cookies to be
re-generated to something not empty.

nixos/modules/services/networking/pleroma.nix

@@ -118,7 +118,7 @@ in {
         # Better be safe than sorry migration-wise.
         ExecStartPre =
           let preScript = pkgs.writers.writeBashBin "pleromaStartPre" ''
-            if [ ! -f "${cookieFile}" ]
+            if [ ! -f "${cookieFile}" ] || [ ! -s "${cookieFile}" ]
             then
               echo "Creating cookie file"
               dd if=/dev/urandom bs=1 count=16 | ${pkgs.hexdump}/bin/hexdump -e '16/1 "%02x"' > "${cookieFile}"