How to exclude the diagonal in a MatrixPlot?

t1 = Table[x^2 + y^2, {x, -3, 3}, {y, -3, 3}];
t2 = t1; t2 = ReplacePart[t2, {i_, i_} :> Null];

Row[{MatrixPlot[t1, ColorFunction -> Hue, ImageSize -> 400],
  MatrixPlot[t2, ColorFunction -> Hue, ImageSize -> 400, ColorRules -> {Null -> None}],
  MatrixPlot[t2 - Min@t1, ColorFunction -> Hue, ImageSize -> 400, ColorRules -> {Null -> None}]}]

Row[{MatrixPlot[t1, ImageSize -> 400],
  MatrixPlot[t2, ImageSize -> 400, ColorRules -> {Null -> None}],
  MatrixPlot[t2 - Min@t1, ImageSize -> 400, ColorRules -> {Null -> None}]}]

Update:

I need a PlotLegend in my plot

t2b = DeleteDuplicates[Sort[Join @@ t2 /. Null -> (min = Min@t1 - 1), Greater]] /. min -> "Null";
mp = MatrixPlot[t2, ImageSize -> 400, ColorRules -> {Null -> None}];
legend = MatrixPlot[List /@ t2b, ColorRules -> {"Null" -> None},
   FrameTicks -> {{None, Transpose[{Range[Length@t2b], t2b}]}, {None, None}},
   PlotRangePadding -> 0, ImageSize -> {60, Automatic}];

Legended[mp, legend]