REF: slice before constructing JoinUnit
I'm seeing small speedups in the case from #50652, not as much as I had hoped for.  The main upside is significantly simplifying JoinUnit.
Thanks @jbrockmendel  It looks like this PR made a change in behaviour (for the best):

```python
import pandas as pd

arr1 = [1.0, '1', 'Allen, Miss. Elisabeth Walton', 'female', 29.0, 0.0, 0.0, '24160', 211.3375, 'B5', 'S', '2', None, 'St Louis, MO']
arr2 = [
    [1.0, '1', 'Allison, Master. Hudson Trevor', 'male', 0.9167, 1.0, 2.0, '113781', 151.55, 'C22 C26', 'S', '11', None, 'Montreal, PQ / Chesterville, ON'],
    [1.0, '0', 'Allison, Miss. Helen Loraine', 'female', 2.0, 1.0, 2.0, '113781', 151.55, 'C22 C26', 'S', None, None, 'Montreal, PQ / Chesterville, ON'],
    [1.0, '0', 'Allison, Mr. Hudson Joshua Creighton', 'male', 30.0, 1.0, 2.0, '113781', 151.55, 'C22 C26', 'S', None, 135.0, 'Montreal, PQ / Chesterville, ON']

pd.concat([pd.DataFrame([arr1]), pd.DataFrame(arr2)], ignore_index=True)
]
```

The previous code would output:

```
    0  1                                     2       3        4    5    6       7         8        9  10    11     12                               13
0  1.0  1         Allen, Miss. Elisabeth Walton  female  29.0000  0.0  0.0   24160  211.3375       B5  S     2   None                     St Louis, MO
1  1.0  1        Allison, Master. Hudson Trevor    male   0.9167  1.0  2.0  113781  151.5500  C22 C26  S    11    NaN  Montreal, PQ / Chesterville, ON
2  1.0  0          Allison, Miss. Helen Loraine  female   2.0000  1.0  2.0  113781  151.5500  C22 C26  S  None    NaN  Montreal, PQ / Chesterville, ON
3  1.0  0  Allison, Mr. Hudson Joshua Creighton    male  30.0000  1.0  2.0  113781  151.5500  C22 C26  S  None  135.0  Montreal, PQ / Chesterville, ON
```

while `main` output:

```
    0  1                                     2       3        4    5    6       7         8        9  10    11     12                               13
0  1.0  1         Allen, Miss. Elisabeth Walton  female  29.0000  0.0  0.0   24160  211.3375       B5  S     2    NaN                     St Louis, MO
1  1.0  1        Allison, Master. Hudson Trevor    male   0.9167  1.0  2.0  113781  151.5500  C22 C26  S    11    NaN  Montreal, PQ / Chesterville, ON
2  1.0  0          Allison, Miss. Helen Loraine  female   2.0000  1.0  2.0  113781  151.5500  C22 C26  S  None    NaN  Montreal, PQ / Chesterville, ON
3  1.0  0  Allison, Mr. Hudson Joshua Creighton    male  30.0000  1.0  2.0  113781  151.5500  C22 C26  S  None  135.0  Montreal, PQ / Chesterville, ON
```

Looking at column 12, the dtype has changed from `object` to `float64`. Somehow, in our specific use case, it is better because we have a more appropriate casting.

However, it introduces an inconsistency between `DataFrame` and `Series`:

```python
In [6]: pd.concat([pd.DataFrame([[None]]), pd.DataFrame([[None], [None], [1.0]])], ignore_index=True)
Out[6]: 
     0
0  NaN
1  NaN
2  NaN
3  1.0

In [7]: pd.concat([pd.Series([None]), pd.Series([None, None, 1.0])], ignore_index=True)
Out[7]: 
0    None
1     NaN
2     NaN
3     1.0
dtype: object
```

I don't know until which point some of the behaviour could be considered as breaking change here. @jbrockmendel might be good to have a follow up with a unit test and whatsnew entry about this change