[SPARK-23328][PYTHON] Disallow default value None in na.replace/replace when 'to_replace' is not a dictionary

## What changes were proposed in this pull request?

This PR proposes to disallow default value None when 'to_replace' is not a dictionary.

It seems weird we set the default value of `value` to `None` and we ended up allowing the case as below:

```python
>>> df.show()
```
```
+----+------+-----+
| age|height| name|
+----+------+-----+
|  10|    80|Alice|
...
```

```python
>>> df.na.replace('Alice').show()
```
```
+----+------+----+
| age|height|name|
+----+------+----+
|  10|    80|null|
...
```

**After**

This PR targets to disallow the case above:

```python
>>> df.na.replace('Alice').show()
```
```
...
TypeError: value is required when to_replace is not a dictionary.
```

while we still allow when `to_replace` is a dictionary:

```python
>>> df.na.replace({'Alice': None}).show()
```
```
+----+------+----+
| age|height|name|
+----+------+----+
|  10|    80|null|
...
```

## How was this patch tested?

Manually tested, tests were added in `python/pyspark/sql/tests.py` and doctests were fixed.

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #20499 from HyukjinKwon/SPARK-19454-followup.