[SPARK-46581][CORE] Update comment on isZero in AccumulatorV2

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

Two changes:
- Update comment on `AccumulatorV2`'s `isZero` to reflect what it actually does.
- Update variable name in `SQLMetrics` to `defaultValidValue` to reflect this

### Why are the changes needed?

`AccumulatorV2`'s `isZero` doesn't do what the comment implies - it actually checks if the accumulator hasn't been updated.

The comment implies that for a `LongAccumulator`, for example, a value of `0` would cause `isZero` to be `true`. But if we were to `add(0)`, then the value would still be `0` but `isZero` would return `false`.

Changing the name of `zeroValue` to `defaultValidValue` to avoid confusion since `isZero` doesn't use `zeroValue` in `SQLMetric`.

Thanks arvindsaik for pointing this out.

### Does this PR introduce _any_ user-facing change?
No.

### How was this patch tested?
Existing tests.

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

Closes #44583 from davintjong-db/sqlmetric-zerovalue-refactor.

Authored-by: Davin Tjong <davin.tjong@databricks.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>

sql/core/src/main/scala/org/apache/spark/sql/execution/metric/SQLMetrics.scala

@@ -40,20 +40,20 @@ import org.apache.spark.util.AccumulatorContext.internOption
 class SQLMetric(
     val metricType: String,
     initValue: Long = 0L,
-    zeroValue: Long = 0L) extends AccumulatorV2[Long, Long] {
-  // initValue defines the initial value of the metric. zeroValue defines the lowest value
-  // considered valid. If a SQLMetric is invalid, it is set to zeroValue upon receiving any
-  // updates, and it also reports zeroValue as its value to avoid exposing it to the user
+    defaultValidValue: Long = 0L) extends AccumulatorV2[Long, Long] {
+  // initValue defines the initial value of the metric. defaultValidValue defines the lowest value
+  // considered valid. If a SQLMetric is invalid, it is set to defaultValidValue upon receiving any
+  // updates, and it also reports defaultValidValue as its value to avoid exposing it to the user
   // programatically.
   //
-  // For many SQLMetrics, we use initValue = -1 and zeroValue = 0 to indicate that the metric is
-  // by default invalid. At the end of a task, we will update the metric making it valid, and the
-  // invalid metrics will be filtered out when calculating min, max, etc. as a workaround for
-  // SPARK-11013.
+  // For many SQLMetrics, we use initValue = -1 and defaultValidValue = 0 to indicate that the
+  // metric is by default invalid. At the end of a task, we will update the metric making it valid,
+  // and the invalid metrics will be filtered out when calculating min, max, etc. as a workaround
+  // for SPARK-11013.
   private var _value = initValue
 
   override def copy(): SQLMetric = {
-    val newAcc = new SQLMetric(metricType, initValue, zeroValue)
+    val newAcc = new SQLMetric(metricType, initValue, defaultValidValue)
     newAcc._value = _value
     newAcc
   }
@@ -63,7 +63,7 @@ class SQLMetric(
   override def merge(other: AccumulatorV2[Long, Long]): Unit = other match {
     case o: SQLMetric =>
       if (o.isValid) {
-        if (!isValid) _value = zeroValue
+        if (!isValid) _value = defaultValidValue
         _value += o.value
       }
     case _ => throw QueryExecutionErrors.cannotMergeClassWithOtherClassError(
@@ -73,14 +73,14 @@ class SQLMetric(
   // This is used to filter out metrics. Metrics with value equal to initValue should
   // be filtered out, since they are either invalid or safe to filter without changing
   // the aggregation defined in [[SQLMetrics.stringValue]].
-  // Note that we don't use zeroValue here since we may want to collect zeroValue metrics
-  // for calculating min, max, etc. See SPARK-11013.
+  // Note that we don't use defaultValidValue here since we may want to collect
+  // defaultValidValue metrics for calculating min, max, etc. See SPARK-11013.
   override def isZero: Boolean = _value == initValue
 
-  def isValid: Boolean = _value >= zeroValue
+  def isValid: Boolean = _value >= defaultValidValue
 
   override def add(v: Long): Unit = {
-    if (!isValid) _value = zeroValue
+    if (!isValid) _value = defaultValidValue
     _value += v
   }
 
@@ -93,8 +93,8 @@ class SQLMetric(
   def +=(v: Long): Unit = add(v)
 
   // _value may be invalid, in many cases being -1. We should not expose it to the user
-  // and instead return zeroValue.
-  override def value: Long = if (!isValid) zeroValue else _value
+  // and instead return defaultValidValue.
+  override def value: Long = if (!isValid) defaultValidValue else _value
 
   // Provide special identifier as metadata so we can tell that this is a `SQLMetric` later
   override def toInfo(update: Option[Any], value: Option[Any]): AccumulableInfo = {