[SPARK-46875][SQL] When the mode is null, a NullPointException sh…

…ould `not` be thrown

### What changes were proposed in this pull request?
The pr aims to provide better prompts when option's `mode` is null.

### Why are the changes needed?
In the original logic, if the mode is null, Spark will throw a `NullPointerException`, which is obviously unfriendly to the user.

```
val cars = spark.read
      .format("csv")
      .options(Map("header" -> "true", "mode" -> null))
      .load(testFile(carsFile))
cars.show(false)
```

Before:
```
Cannot invoke "String.toUpperCase(java.util.Locale)" because "mode" is null
java.lang.NullPointerException: Cannot invoke "String.toUpperCase(java.util.Locale)" because "mode" is null
	at org.apache.spark.sql.catalyst.util.ParseMode$.fromString(ParseMode.scala:50)
	at org.apache.spark.sql.catalyst.csv.CSVOptions.$anonfun$parseMode$1(CSVOptions.scala:105)
	at scala.Option.map(Option.scala:242)
	at org.apache.spark.sql.catalyst.csv.CSVOptions.<init>(CSVOptions.scala:105)
	at org.apache.spark.sql.catalyst.csv.CSVOptions.<init>(CSVOptions.scala:49)
	at org.apache.spark.sql.execution.datasources.csv.CSVFileFormat.inferSchema(CSVFileFormat.scala:60)
```

After:
It will fall back to `PermissiveMode` mode and then display the data normally, as shown below:
```
18:54:06.727 WARN org.apache.spark.sql.catalyst.util.ParseMode: mode is null and not a valid parse mode. Using PERMISSIVE.

+----+-----+-----+----------------------------------+-----+
|year|make |model|comment                           |blank|
+----+-----+-----+----------------------------------+-----+
|2012|Tesla|S    |No comment                        |NULL |
|1997|Ford |E350 |Go get one now they are going fast|NULL |
|2015|Chevy|Volt |NULL                              |NULL |
+----+-----+-----+----------------------------------+-----+
```

### Does this PR introduce _any_ user-facing change?
Yes, When `mode` is null, it fallback to `PermissiveMode ` instead of throwing a `NullPointerException`.

### How was this patch tested?
- Add new UT.
- Pass GA.

### Was this patch authored or co-authored using generative AI tooling?
No.

Closes apache#44900 from panbingkun/SPARK-46875.

Authored-by: panbingkun <panbingkun@baidu.com>
Signed-off-by: Max Gekk <max.gekk@gmail.com>

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/ParseMode.scala

@@ -47,12 +47,17 @@ object ParseMode extends Logging {
   /**
    * Returns the parse mode from the given string.
    */
-  def fromString(mode: String): ParseMode = mode.toUpperCase(Locale.ROOT) match {
-    case PermissiveMode.name => PermissiveMode
-    case DropMalformedMode.name => DropMalformedMode
-    case FailFastMode.name => FailFastMode
-    case _ =>
-      logWarning(s"$mode is not a valid parse mode. Using ${PermissiveMode.name}.")
-      PermissiveMode
+  def fromString(mode: String): ParseMode = Option(mode).map {
+    v => v.toUpperCase(Locale.ROOT) match {
+      case PermissiveMode.name => PermissiveMode
+      case DropMalformedMode.name => DropMalformedMode
+      case FailFastMode.name => FailFastMode
+      case _ =>
+        logWarning(s"$v is not a valid parse mode. Using ${PermissiveMode.name}.")
+        PermissiveMode
+    }
+  }.getOrElse {
+    logWarning(s"mode is null and not a valid parse mode. Using ${PermissiveMode.name}.")
+    PermissiveMode
   }
 }

sql/core/src/test/scala/org/apache/spark/sql/execution/datasources/csv/CSVSuite.scala

@@ -348,6 +348,16 @@ abstract class CSVSuite
     }
   }
 
+  test("when mode is null, will fall back to PermissiveMode mode") {
+    val cars = spark.read
+      .format("csv")
+      .options(Map("header" -> "true", "mode" -> null))
+      .load(testFile(carsFile))
+    assert(cars.collect().length == 3)
+    assert(cars.select("make").collect() sameElements
+      Array(Row("Tesla"), Row("Ford"), Row("Chevy")))
+  }
+
   test("test for blank column names on read and select columns") {
     val cars = spark.read
       .format("csv")