Add is_empty function

Add a function to check if an object is empty. An object is considered empty if it is nil, a table with out any items (key, value pairs or indexes), or a string with no content ("").

Ex:

```
t = {}
s = ""
if utils.is_empty(t) or utils.is_empty(s) then
...
```

Further reduces the need for statements such as
```
if not t and not next(t) then
...
```