early returns from function doesnt add history column

The following discussion from !338 (merged) should be addressed:

• @schaefed started a discussion: (+5 comments)

  This is a general concern with our flag handling and related to the code comment:

  Functions with an early return like:

  if data[field].empty:
      return data, flags

  don't set any flags at all and therefore leave no traits in the History. This unfortunate as some flagging schemes depend on a complete history. I fixed the early-returning functions I found with something like:

  if data[field].empty:
      flags[field] = UNFLAGGED
      return data, flags

  which is ... IDK, at least prone to errors. So maybe we should handle this here with something like:

  if new_history.empty:
      new_history = History(old_history.index).append(UNFLAGGED)

  (or maybe something more straightforward...). What do you think?