export is_table_empty from utils module

docker/nginx/libs/utils.lua

@@ -1,7 +1,7 @@
 local _M = {}
 
 -- utility function for checking if table is empty
-function is_table_empty(check)
+function _M.is_table_empty(check)
     -- bind next to local variable to achieve ultimate efficiency
     -- https://stackoverflow.com/a/1252776
     local next = next