[SPARK-57726][SQL] Fix NPE in AttributeReference.hashCode when the attribute name is null

### What changes were proposed in this pull request?

`AttributeReference.hashCode` computes `name.hashCode()` directly:

```scala
override def hashCode: Int = {
  var h = 17
  h = h * 37 + name.hashCode()   // NPE if name == null
  ...
}
```

This throws a `NullPointerException` when the attribute has a null name. This PR makes the name contribution null-safe using `Objects.hashCode(name)` (`java.util.Objects` is already imported).

### Why are the changes needed?

`AttributeReference.equals` already treats the name nullably (`name == ar.name`), but `hashCode` does not. An `AttributeReference` can carry a null name (e.g. a `StructField` permits a null name and it flows into `AttributeReference` via `DataTypeUtils.toAttribute`). When such an attribute is placed in a hash-based collection (`HashSet`/`HashMap`, or via `distinct`/`toSet`), `hashCode` throws an NPE.

This was noticed during review of #56831 (SPARK-57725), which fixes a related but distinct null-name NPE in `AttributeSeq`'s case-insensitive name maps. The two issues are independent and are addressed separately.

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

Added a test in `NamedExpressionSuite` asserting that `AttributeReference.hashCode` does not throw on a null-named attribute and that the equals/hashCode contract holds:

```
build/sbt 'catalyst/testOnly *NamedExpressionSuite'
```

### Was this patch authored or co-authored using generative AI tooling?

Generated-by: Cursor

Closes #56832 from MaxGekk/fix-attrref-hashcode-npe.

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

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/namedExpressions.scala

diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/namedExpressions.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/namedExpressions.scala
index d27f140..c7a2165 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/namedExpressions.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/namedExpressions.scala
@@ -312,7 +312,9 @@
   override def hashCode: Int = {
     // See http://stackoverflow.com/questions/113511/hash-code-implementation
     var h = 17
-    h = h * 37 + name.hashCode()
+    // Use Objects.hashCode to stay null-safe: an AttributeReference can carry a null name
+    // (e.g. from a StructField built with a null name), and equals already treats name nullably.
+    h = h * 37 + Objects.hashCode(name)
     h = h * 37 + dataType.hashCode()
     h = h * 37 + nullable.hashCode()
     h = h * 37 + metadata.hashCode()

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/NamedExpressionSuite.scala

diff --git a/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/NamedExpressionSuite.scala b/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/NamedExpressionSuite.scala
index 3e6f40f..362313c 100644
--- a/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/NamedExpressionSuite.scala
+++ b/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/NamedExpressionSuite.scala
@@ -64,4 +64,17 @@
     assert(!alias.metadata.contains(nonInheritableMetadataKey))
     assert(alias.metadata.contains("key"))
   }
+
+  test("SPARK-57726: AttributeReference.hashCode is null-safe when the name is null") {
+    // An AttributeReference can carry a null name (e.g. from a StructField built with a null
+    // name). hashCode must not throw a NullPointerException so the attribute can be used in
+    // hash-based collections; equals is already null-safe on the name.
+    val exprId = NamedExpression.newExprId
+    val a1 = AttributeReference(null, IntegerType)(exprId)
+    val a2 = AttributeReference(null, IntegerType)(exprId)
+    a1.hashCode()
+    assert(a1 == a2)
+    assert(a1.hashCode() == a2.hashCode())
+    assert(Set(a1).contains(a2))
+  }
 }