### What changes were proposed in this pull request?
This PR proposes two things:
1. Convert `null` to `string` type during schema inference of `schema_of_json` as JSON datasource does. This is a bug fix as well because `null` string is not the proper DDL formatted string and it is unable for SQL parser to recognise it as a type string. We should match it to JSON datasource and return a string type so `schema_of_json` returns a proper DDL formatted string.
2. Let `schema_of_json` respect `dropFieldIfAllNull` option during schema inference.
### Why are the changes needed?
To let `schema_of_json` return a proper DDL formatted string, and respect `dropFieldIfAllNull` option.
### Does this PR introduce any user-facing change?
Yes, it does.
```scala
import collection.JavaConverters._
import org.apache.spark.sql.functions._
spark.range(1).select(schema_of_json(lit("""{"id": ""}"""))).show()
spark.range(1).select(schema_of_json(lit("""{"id": "a", "drop": {"drop": null}}"""), Map("dropFieldIfAllNull" -> "true").asJava)).show(false)
```
**Before:**
```
struct<id:null>
struct<drop:struct<drop:null>,id:string>
```
**After:**
```
struct<id:string>
struct<id:string>
```
### How was this patch tested?
Manually tested, and unittests were added.
Closes#27854 from HyukjinKwon/SPARK-31065.
Authored-by: HyukjinKwon <gurwls223@apache.org>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>
815c7929c2