cubicweb #2036499 wrong rset description in some case of UNION queries [resolved]

because it only consider a sample select node / solution to compute it while those particular one may miss information foundable in others

prioritynormal
typebug
done in3.13.9
load0.100
load left0.000
closed by#d43569aaf5d6 [rset] don't only consider a sample select node / solution to compute rset description. Closes #2036499