Don't return invalid XML from paraminfo when one of parameter values is null

Don't return invalid XML from paraminfo when one of parameter values is null

The module setglobalaccountstatus currently has null
as one of the values for one of its parameters,
which causes paraminfo to return invalid XML (unclosed tag).

I believe values shouldn't be null, but paraminfo
shouldn't return invalid XML even when they are.

This problem occured only with values that go into element content,
those that are rendered as attributes were already fine.

This change modifies ApiFormatXml, so it affects all modules.

Change-Id: Ibf5c329e7bfa375b06f0976ebb9e449f2cb1c927