[SPARK-46937][SQL][FOLLOWUP] Properly check registered function replacement

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

A followup of #44976 . `ConcurrentHashMap#put` has a different semantic than the scala map, and it returns null if the key is new. We should update the checking code accordingly.

### Why are the changes needed?

avoid wrong warning messages

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

no

### How was this patch tested?

manual

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

no

Closes #46876 from cloud-fan/log.

Authored-by: Wenchen Fan <[email protected]>
Signed-off-by: Kent Yao <[email protected]>

Author: cloud-fan

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/FunctionRegistry.scala

@@ -222,7 +222,7 @@ trait SimpleFunctionRegistryBase[T] extends FunctionRegistryBase[T] with Logging
       builder: FunctionBuilder): Unit = {
     val newFunction = (info, builder)
     functionBuilders.put(name, newFunction) match {
-      case previousFunction if previousFunction != newFunction =>
+      case previousFunction if previousFunction != null =>
         logWarning(log"The function ${MDC(FUNCTION_NAME, name)} replaced a " +
           log"previously registered function.")
       case _ =>