Themes need THEMES_CODE_DIR defined to something that exists

It doesn't matter what it is or what is in it, it just needs to exist
on disk so that platform code can touch it.

util/jenkins/check-migrations.sh

@@ -57,6 +57,7 @@ extra_var_args+=" -e edxapp_app_dir=${WORKSPACE}"
 extra_var_args+=" -e edxapp_code_dir=${WORKSPACE}/edx-platform"
 extra_var_args+=" -e edxapp_user=jenkins"
 extra_var_args+=" -e EDXAPP_CFG_DIR=${WORKSPACE}"
+extra_var_args+=" -e THEMES_CODE_DIR=${WORKSPACE}"
 
 # Run migrations and replace literal '\n' with actual newlines to make the output
 # easier to read