fix(library/st_task_queue): handle null dependencies
fix(library/st_task_queue): handle null dependencies

leodemoura/lean,avigad/lean,fpvandoorn/lean,digama0/lean,leanprover/lean,dselsam/lean,leanprover/lean,johoelzl/lean,fgdorais/lean,leanprover/lean,fgdorais/lean,johoelzl/lean,leanprover/lean,digama0/lean,leanprover-community/lean,fgdorais/lean,rlewis1988/lean,johoelzl/lean,Kha/lean,leanprover/lean,rlewis1988/lean,Kha/lean,dselsam/lean,leanprover-community/lean,avigad/lean,rlewis1988/lean,Kha/lean,digama0/lean,dselsam/lean,leanprover-community/lean,leodemoura/lean,rlewis1988/lean,digama0/lean,johoelzl/lean,digama0/lean,Kha/lean,leanprover/lean,digama0/lean,leodemoura/lean,leanprover-community/lean,avigad/lean,avigad/lean,leanprover-community/lean,dselsam/lean,fgdorais/lean,johoelzl/lean,fpvandoorn/lean,johoelzl/lean,leodemoura/lean,fpvandoorn/lean,leodemoura/lean,dselsam/lean,fpvandoorn/lean,rlewis1988/lean,Kha/lean,Kha/lean,leodemoura/lean,fpvandoorn/lean,leanprover-community/lean,dselsam/lean,fpvandoorn/lean,avigad/lean,avigad/lean,fgdorais/lean,fgdorais/lean,rlewis1988/lean