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?

Edited Nov 25, 2021 by David Schäfer