Fix File.join with separator path component

[icyleaf]

Simliar #5915.

It throws an exception if only separator in the middle component, (for example File.join("foo", "/", "bar"))

Unhandled exception: Index out of bounds (IndexError)
  from Slice(UInt8)@Slice(T)#pointer<Int32>:Pointer(UInt8)
  from Slice(UInt8)@Slice(T)#copy_to<Pointer(UInt8), Int32>:Pointer(UInt8)
  from String::Builder#write<Slice(UInt8)>:Nil
  from File::join<Tuple(String, String, String)>:String
  from File::join<String, String, String>:String
  from __icr_exec__:String
  from __crystal_main
  from Crystal::main_user_code<Int32, Pointer(Pointer(UInt8))>:Nil
  from Crystal::main<Int32, Pointer(Pointer(UInt8))>:Int32
  from main

[bcardiff]

Maybe we should wait on this after #5635 since the code is moving to other file.

[straight-shoota]

This is already fixed in #5635, but it doesn't hurt to merge.

[asterite]

It would be really nice to add comments inside the implementation. It's becoming really obscure.