treat error in get_text_attributes_at_offset as no attributes everywhere
When GetAttributeRun, for example, is not implemented, or fails otherwise for a text, we know of no attributes for it, and do so for the complete content of it.
Thus, it makes sense to claim said range – the complete text – has no attributes.
That makes users like splitSubstringByLanguage still use the string, even though the attributes are not known.
Note: This will only work after #477 (closed) being fixed.