feat(kernel/expr): change dummy expression used in default constructor
feat(kernel/expr): change dummy expression used in default constructor

johoelzl/lean,fgdorais/lean,digama0/lean,levnach/lean,UlrikBuchholtz/lean,dselsam/lean,UlrikBuchholtz/lean,leanprover/lean,soonhokong/lean,Kha/lean,johoelzl/lean,fpvandoorn/lean,leanprover-community/lean,johoelzl/lean,fpvandoorn/lean,soonhokong/lean-osx,levnach/lean,leanprover-community/lean,leanprover/lean,fgdorais/lean,dselsam/lean,soonhokong/lean,avigad/lean,UlrikBuchholtz/lean,fgdorais/lean,leanprover-community/lean,leanprover/lean,soonhokong/lean-windows,fpvandoorn/lean,javra/lean,avigad/lean,leanprover/lean,levnach/lean,Kha/lean,avigad/lean,avigad/lean,rlewis1988/lean,fpvandoorn/lean,fpvandoorn/lean,soonhokong/lean-windows,fgdorais/lean,fpvandoorn/lean2,leodemoura/lean,johoelzl/lean,leanprover/lean,leanprover-community/lean,rlewis1988/lean,soonhokong/lean,levnach/lean,digama0/lean,fgdorais/lean,soonhokong/lean-osx,avigad/lean,fpvandoorn/lean,soonhokong/lean,Kha/lean,digama0/lean,johoelzl/lean,rlewis1988/lean,fpvandoorn/lean2,rlewis1988/lean,Kha/lean,fgdorais/lean,soonhokong/lean-osx,avigad/lean,Kha/lean,digama0/lean,rlewis1988/lean,Kha/lean,leodemoura/lean,leodemoura/lean,fpvandoorn/lean2,digama0/lean,leodemoura/lean,leanprover-community/lean,javra/lean,javra/lean,soonhokong/lean-osx,soonhokong/lean,javra/lean,soonhokong/lean-windows,dselsam/lean,dselsam/lean,leanprover-community/lean,leanprover/lean,rlewis1988/lean,javra/lean,levnach/lean,soonhokong/lean-windows,UlrikBuchholtz/lean,dselsam/lean,digama0/lean,johoelzl/lean,soonhokong/lean-osx,leodemoura/lean,leodemoura/lean,fpvandoorn/lean2,dselsam/lean,UlrikBuchholtz/lean,fpvandoorn/lean2,soonhokong/lean-windows