BUG: DataFrame.append with timedelta64
- [ ] closes #xxxx
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry

AFAICT several of the existing tests are just wrong, xref #39122 cc @jorisvandenbossche 

Fixes (but havent yet added a test for) https://github.com/pandas-dev/pandas/issues/39037#issuecomment-756748197


Can you summarize the behaviour that is changed / bug that is fixed? > Can you summarize the behaviour that is changed / bug that is fixed?

```
df = DataFrame({"a": pd.array([1], dtype="Int64")})
other = DataFrame({"a": [np.timedelta64("NaT", "ns")]})
result = df.append(other, ignore_index=True)

>>> result
      a
0     1
1  <NA>
>>> result.dtypes
a    Int64
dtype: object
```

```
df = DataFrame(columns=["a"]).astype("Int64")
other = DataFrame({"a": [np.timedelta64(1, "ns")]})

>>> result = df.append(other, ignore_index=True)
ValueError: Wrong number of dimensions. values.ndim != ndim [1 != 2]
``` @jorisvandenbossche helpful comments, thank you.  i found a nice way to retain the None/nan behavior while fixing the original bug: supplementing JoinUnit.is_na with a check that it is a compatible NA. @jorisvandenbossche i think your comments have all been addressed, lmk if i missed anything @jorisvandenbossche gentle ping before your day ends Can you add a whatsnew? will do, thanks