maintainers: remove null fields

lib/maintainers-list.nix

···
     name = "Pascal Bach";
   };
   backuitist = {
-    email = null;
+    email = "[e-mail]";
     github = "backuitist";
     name = "Bruno Bieth";
   };
···
     name = "Pierre Carrier";
   };
   periklis = {
-    email = null;
+    email = "[e-mail]";
     github = "periklis";
-    name = "[e-mail]";
+    name = "Periklis Tsirakidis";
   };
   pesterhazy = {
     email = "[e-mail]";