[krbdev.mit.edu #5742] NIM: FEATURE: LIB: the khc_read_multi_string() etc APIs should handle NULL returns

[Jeffrey Altman via RT]

If an API is returning a multi string and the string list being
returned is empty, we should use a separate return value.  This
way, callers won't have to allocate two words for the double NULL
terminator only to find that the multi string doesn't actually
contain any strings.