Check if env var is empty (if it does exists but unset)

Idea from https://github.com/lsangild/ITAMS/blob/d23841b09ff5eab38dd533aa2428ca9f25557d24/Report/texsetup/lua/utility.lua

lib/lua/envvar_newcommands.lua

@@ -30,7 +30,7 @@ local macros_to_envvars = {
 
 for macro_name, env_var in pairs(macros_to_envvars) do
     local content = os.getenv(env_var)
-    if content == nil then
+    if content == nil or content == "" then
         content = missing
     end
     --[[