Use `nothing` for the missing online/offline trees

src/root/root.jl

 struct ROOTFile
     _fobj::UnROOT.ROOTFile
-    online::Union{OnlineTree, Missing}
-    offline::Union{OfflineTree, Missing}
+    online::Union{OnlineTree, Nothing}
+    offline::Union{OfflineTree, Nothing}
 
     function ROOTFile(filename::AbstractString)
         customstructs = Dict(
@@ -13,17 +13,17 @@ struct ROOTFile
         )
         fobj = UnROOT.ROOTFile(filename, customstructs=customstructs)
         tpath_offline = ROOT.TTREE_OFFLINE_EVENT
-        offline = tpath_offline ∈ keys(fobj) ? OfflineTree(fobj) : missing
+        offline = tpath_offline ∈ keys(fobj) ? OfflineTree(fobj) : nothing
         tpath_online = ROOT.TTREE_ONLINE_EVENT
-        online = tpath_online ∈ keys(fobj) ? OnlineTree(fobj) : missing
+        online = tpath_online ∈ keys(fobj) ? OnlineTree(fobj) : nothing
         new(fobj, online, offline)
     end
 end
 Base.close(f::ROOTFile) = close(f._fobj)
 function Base.show(io::IO, f::ROOTFile)
     s = String[]
-    !ismissing(f.online) && push!(s, "$(f.online)")
-    !ismissing(f.offline) && push!(s, "$(f.offline)")
+    !isnothing(f.online) && push!(s, "$(f.online)")
+    !isnothing(f.offline) && push!(s, "$(f.offline)")
     info = join(s, ", ")
     print(io, "ROOTFile{$info}")
 end