asso = <|"x" -> {
    <|"a" -> 5, "b" -> "y", "c" -> {5, 6, 7}|>,
    <|"a" -> 6, "b" -> "z", "c" -> {}|>}
|>

asso // Query["x", Select[#c === {} &], Framed@*"b"]

{Framed["z"]}

asso // Query[
  {"x"},
  MapAt[
    MapAt[Framed, #, "b"] &,
    #,
    Position[#, _?(Function[asso, asso["c"] === {}])]] &
  ]

bSel = MapAt[Framed, #, "b"] &;
cPos = Position[#, _?(#c === {} &)] &
cSel = MapAt[bSel, #, cPos[#]] &
cSel /@ asso