### What changes were proposed in this pull request?
1. Recognize `null` while parsing partition specs, and put `null` instead of `"null"` as partition values.
2. For V1 catalog: replace `null` by `__HIVE_DEFAULT_PARTITION__`.
3. For V2 catalogs: pass `null` AS IS, and let catalog implementations to decide how to handle `null`s as partition values in spec.
### Why are the changes needed?
Currently, `null` in partition specs is recognized as the `"null"` string which could lead to incorrect results, for example:
```sql
spark-sql> CREATE TABLE tbl5 (col1 INT, p1 STRING) USING PARQUET PARTITIONED BY (p1);
spark-sql> INSERT INTO TABLE tbl5 PARTITION (p1 = null) SELECT 0;
spark-sql> SELECT isnull(p1) FROM tbl5;
false
```
Even we inserted a row to the partition with the `null` value, **the resulted table doesn't contain `null`**.
### Does this PR introduce _any_ user-facing change?
Yes. After the changes, the example above works as expected:
```sql
spark-sql> SELECT isnull(p1) FROM tbl5;
true
```
### How was this patch tested?
1. By running the affected test suites `SQLQuerySuite`, `AlterTablePartitionV2SQLSuite` and `v1/ShowPartitionsSuite`.
2. Compiling by Scala 2.13:
```
$ ./dev/change-scala-version.sh 2.13
$ ./build/sbt -Pscala-2.13 compile
```
Closes#30538 from MaxGekk/partition-spec-value-null.
Authored-by: Max Gekk <max.gekk@gmail.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>
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