doc: array_length produces null instead of 0

Hi,

Per discussion here:

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.

David J.