[SPARK-49476][SQL] Fix nullability of base64 function

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

Fix the nullability of the `Base64` expression to be based on the child's nullability, and not always be nullable.

### Why are the changes needed?

apache#47303 had a side effect of changing the nullability by the switch to using `StaticInvoke`. This was also backported to Spark 3.5.2 and caused schema mismatch errors for stateful streams when we upgraded. This restores the previous behavior which is supported by StaticInvoke through the `returnNullable` argument. If the child is non-nullable, we know the result will be non-nullable.

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

Restores the nullability of the `Base64` expression to what is was in Spark 3.5.1 and earlier.

### How was this patch tested?

New UT

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

No

Closes apache#47941 from Kimahriman/base64-nullability.

Lead-authored-by: Adam Binford <[email protected]>
Co-authored-by: Maxim Gekk <[email protected]>
Signed-off-by: Max Gekk <[email protected]>

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

@@ -2714,7 +2714,8 @@ case class Base64(child: Expression, chunkBase64: Boolean)
     dataType,
     "encode",
     Seq(child, Literal(chunkBase64, BooleanType)),
-    Seq(BinaryType, BooleanType))
+    Seq(BinaryType, BooleanType),
+    returnNullable = false)
 
   override def toString: String = s"$prettyName($child)"
 

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

@@ -467,6 +467,13 @@ class StringExpressionsSuite extends SparkFunSuite with ExpressionEvalHelper {
     val b = $"b".binary.at(0)
     val bytes = Array[Byte](1, 2, 3, 4)
 
+    assert(!Base64(Literal(bytes)).nullable)
+    assert(Base64(Literal.create(null, BinaryType)).nullable)
+    assert(Base64(Literal(bytes).castNullable()).nullable)
+    assert(!UnBase64(Literal("AQIDBA==")).nullable)
+    assert(UnBase64(Literal.create(null, StringType)).nullable)
+    assert(UnBase64(Literal("AQIDBA==").castNullable()).nullable)
+
     checkEvaluation(Base64(Literal(bytes)), "AQIDBA==", create_row("abdef"))
     checkEvaluation(Base64(UnBase64(Literal("AQIDBA=="))), "AQIDBA==", create_row("abdef"))
     checkEvaluation(Base64(UnBase64(Literal(""))), "", create_row("abdef"))