This function takes a series of strings as arguments. It will test if
each one is the null string (""). If not, it will return that string 
and stop. If it is null, it will continue on to the next one, and test
that. This will continue until either one of the strings is non-null, or
it reaches the last argument.  Arguments after that are not evaluated.

@strcheck("", "", "a")
@strcheck(@getfield("123", "va"), @getfield("123", "vb"), "Not found.")