gnumeric-reader: Avoid convert_xml_string_to_value segfault on NULL string

gnumeric-reader: Avoid convert_xml_string_to_value segfault on NULL string

If the 'xv' argument is null, the previous version of this code would
still dereference it.  This commit rewrites it to instead just set the
value as missing and avoid the segfault.  It also takes advantage of the
fairly recently added function value_copy_str_rpad() to eliminate some
code.