cubicweb #2126364 Make to easier to override empty rset behaviour of EntityStartupView [validation pending]

We should be able to easily display some message when the rset is empty (patch coming).

prioritynormal
typeenhancement
done in3.14.3
load0.100
load left0.000
closed by#b3e87e6865ef [base views] make it easier to override EntityStartupView behaviour when the rset is empty; closes #2126364