Modify stdnse.output_table to handle empty values better

Two changes here, both minor. First, explicitly assigning a new key to
nil does not add the key to the ordered set of keys. This better
emulates the behavior of regular tables.

> o = stdnse.output_table()
> o["test"] = nil

This previously resulted in output like this:

|_ test: nil

Now it simply omits the "test:" key.

Second, I needed a way to tell whether an output table was empty or not.
Since Lua's next() function doesn't call the __pairs metamethod, it was
always returning nil. Instead, I used the __call metamethod, since it
had the least preexisting semantic meaning:

> o = stdnse.output_table()
> =o()
false
> o["test"] = 1
> =o()
true
Structured XML output for xmpp-info

This involves some reordering and renaming of fields from the previous
output, but hopefully more readable (less vertical whitespace). This
commit depends on the changes to stdnse.output_table to ignore assigning
new keys to nil and to use the __call metamethod to mean has_contents()