Re: doc: array_length produces null instead of 0

Hi David,

> Per discussion here:
>
> https://www.postgresql.org/message-id/163636931138.8076.5140809232053731248%40wrigleys.postgresql.org
>
> We can now easily document the array_length behavior of returning null instead of zero for an empty array/dimension.
>
> I added an example to the json_array_length function to demonstrate that it does return 0 as one would expect, but contrary to the SQL array behavior.
>
> I did not bother to add examples to the other half dozen or so "_length" functions that all produce 0 as expected.  Just the surprising case and the adjacent one.

Good catch.

+        <literal>array_length(array[], 1)</literal>
+        <returnvalue>NULL</returnvalue>

One tiny nitpick I have is that this example will not work if used
literally, as is:

```
=# select array_length(array[], 1);
ERROR:  cannot determine type of empty array
LINE 1: select array_length(array[], 1);
```

Maybe it's worth using `array_length(array[] :: int[], 1)` instead.