[branch-54] fix: read Hive's null partition placeholder as NULL (backport #26310) - #27308

## Which issue does this PR close?

- working towards apache#18083

## Rationale for this change

Hive compliant writers put the rows whose partition value is null into a
directory named `__HIVE_DEFAULT_PARTITION__` (Hive's
`hive.exec.default.partition.name`). `ListingTable` casts that directory
name straight to the partition column's type, so a table with a null
partition cannot be queried at all unless the partition column happens
to be a string:

```sql
> create external table t (id int, d date) stored as parquet partitioned by (d) location 'data/';
> select * from t;
Arrow error: Cast error: Cannot cast string '__HIVE_DEFAULT_PARTITION__' to value of Date32 type
```

For a string partition column the query runs, but the value comes back
as the literal `__HIVE_DEFAULT_PARTITION__` rather than NULL, and `WHERE
d IS NULL` never matches it because partition columns are declared
non-nullable. Hive-written tables are a common input, and both Spark and
Impala read these values as NULL.

We are not touching writes in this PR - just reads.
We also haven’t fixed the /date=/a.parquet problem here either.

## What changes are included in this PR?

1. A partition value equal to the placeholder is read as a typed NULL
rather than parsed
   (`helpers::try_into_partitioned_file`).
2. `ListingTable` declares partition columns nullable, in both the table
schema and the file
source's `TableSchema`, so the NULL can be represented and `IS NULL`
selects the null partition.
3. A new option,
`datafusion.execution.listing_table_hive_default_partition_name`,
defaulting to
`__HIVE_DEFAULT_PARTITION__`, so a table written with a different
`hive.exec.default.partition.name` can be read. The empty name turns the
mapping off and reads the directory name as the value. The default is
also exposed as`datafusion_common::config::HIVE_DEFAULT_PARTITION_NAME`.
4. `filter_partitioned_file` no longer treats a NULL predicate result as
a match. That is reachable now that a partition value can be NULL
5. Claude caught this one not me: In the generated `configs.md`,
defaults containing `_` or `*` are shown as code. Markdown would
otherwise read `__HIVE_DEFAULT_PARTITION__` as emphasis and render it as
bold text; this also backticks the two existing such defaults,
`nulls_max` and `start_offset`.

## What is the testing strategy for this PR?

Unit tests and slt tests added.

## Are there any user-facing changes?
partition columns are now forced to be nullable
listing_table_hive_default_partition_name added

(cherry picked from commit 407bef6)