cubicweb #1625203 [facets] fix RangeFacet.get_widget [resolved]

the facet currently returns a widget for a 1-value vocabulary, which is quite not enough (2 is the minimum that makes sense)

prioritynormal
typebug
done in3.12.3
load0.200
load left0.000
closed by#f20fc41e3f7d [facets] fix RangeFacet get_widget: do no return a widget unless there are at least 2 values; closes #1625203