[Standalone] Introduce FileAction.tagsOrEmpty (#3132)

## Description
This PR introduces Introduce `FileAction.tagsOrEmpty` to factor out the
common pattern `Option(tags).getOrElse(Map.empty)`.

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

## Does this PR introduce _any_ user-facing changes?
No

Signed-off-by: Sergiu Pocol <sergiu.pocol@databricks.com>

spark/src/main/scala/org/apache/spark/sql/delta/actions/actions.scala

@@ -596,6 +596,10 @@ sealed trait FileAction extends Action {
   @JsonIgnore
   def estLogicalFileSize: Option[Long]
 
+  /** Returns [[tags]] or an empty Map if null */
+  @JsonIgnore
+  def tagsOrEmpty: Map[String, String] = Option(tags).getOrElse(Map.empty[String, String])
+
   /**
    * Return tag value if tags is not null and the tag present.
    */
@@ -788,16 +792,15 @@ case class AddFile(
 
 
   def copyWithTags(newTags: Map[String, String]): AddFile =
-    copy(tags = Option(tags).getOrElse(Map.empty) ++ newTags)
-
+    copy(tags = tagsOrEmpty ++ newTags)
 
   def tag(tag: AddFile.Tags.KeyType): Option[String] = getTag(tag.name)
 
   def longTag(tagKey: AddFile.Tags.KeyType): Option[Long] =
     tag(tagKey).map(_.toLong)
 
   def copyWithTag(tag: AddFile.Tags.KeyType, value: String): AddFile =
-    copy(tags = Option(tags).getOrElse(Map.empty) + (tag.name -> value))
+    copy(tags = tagsOrEmpty + (tag.name -> value))
 
   def copyWithoutTag(tag: AddFile.Tags.KeyType): AddFile = {
     if (tags == null) {
@@ -916,13 +919,13 @@ case class RemoveFile(
    * Create a copy with the new tag. `extendedFileMetadata` is copied unchanged.
    */
   def copyWithTag(tag: String, value: String): RemoveFile = copy(
-    tags = Option(tags).getOrElse(Map.empty) + (tag -> value))
+    tags = tagsOrEmpty + (tag -> value))
 
   /**
    * Create a copy without the tag.
    */
   def copyWithoutTag(tag: String): RemoveFile =
-    copy(tags = Option(tags).getOrElse(Map.empty) - tag)
+    copy(tags = tagsOrEmpty - tag)
 
   @JsonIgnore
   override def getFileSize: Long = size.getOrElse(0L)

spark/src/main/scala/org/apache/spark/sql/delta/zorder/ZCubeInfo.scala

@@ -98,8 +98,7 @@ object ZCubeInfo extends DeltaCommand {
    * Update the given file's metadata to make it part of the given zCubeInfo.
    */
   def setForFile(file: AddFile, zCubeInfo: ZCubeInfo): AddFile = {
-    val oldTags = Option(file.tags).getOrElse(Map.empty)
-    val newTags = oldTags ++ ZCubeInfo.toAddFileTags(zCubeInfo)
+    val newTags = file.tagsOrEmpty ++ ZCubeInfo.toAddFileTags(zCubeInfo)
     file.copy(tags = newTags)
   }