of the lazy instantiation rule this value was not actually written, but
if lazy instantiation is switched on, then this causes that all crawl
starts delete all crawl-start-hosts completely because this looks for
filled error reasons.
if(allAttr||contains(CollectionSchema.failreason_s))add(doc,CollectionSchema.failreason_s,"");// overwrite a possible fail reason (in case that there was a fail reason before)