Core: Serialize null when there is no current snapshot (apache#11560)

Author: Fokko

core/src/main/java/org/apache/iceberg/TableMetadataParser.java

@@ -214,9 +214,11 @@ public static void toJson(TableMetadata metadata, JsonGenerator generator) throw
     // write properties map
     JsonUtil.writeStringMap(PROPERTIES, metadata.properties(), generator);
 
-    generator.writeNumberField(
-        CURRENT_SNAPSHOT_ID,
-        metadata.currentSnapshot() != null ? metadata.currentSnapshot().snapshotId() : -1);
+    if (metadata.currentSnapshot() != null) {
+      generator.writeNumberField(CURRENT_SNAPSHOT_ID, metadata.currentSnapshot().snapshotId());
+    } else {
+      generator.writeNullField(CURRENT_SNAPSHOT_ID);
+    }
 
     toJson(metadata.refs(), generator);
 

core/src/test/java/org/apache/iceberg/rest/responses/TestLoadTableResponseParser.java

@@ -107,7 +107,7 @@ public void roundTripSerde() {
                 + "      \"fields\" : [ ]\n"
                 + "    } ],\n"
                 + "    \"properties\" : { },\n"
-                + "    \"current-snapshot-id\" : -1,\n"
+                + "    \"current-snapshot-id\" : null,\n"
                 + "    \"refs\" : { },\n"
                 + "    \"snapshots\" : [ ],\n"
                 + "    \"statistics\" : [ ],\n"
@@ -180,7 +180,7 @@ public void roundTripSerdeWithConfig() {
                 + "      \"fields\" : [ ]\n"
                 + "    } ],\n"
                 + "    \"properties\" : { },\n"
-                + "    \"current-snapshot-id\" : -1,\n"
+                + "    \"current-snapshot-id\" : null,\n"
                 + "    \"refs\" : { },\n"
                 + "    \"snapshots\" : [ ],\n"
                 + "    \"statistics\" : [ ],\n"
@@ -283,7 +283,7 @@ public void roundTripSerdeWithCredentials() {
                 + "      \"fields\" : [ ]\n"
                 + "    } ],\n"
                 + "    \"properties\" : { },\n"
-                + "    \"current-snapshot-id\" : -1,\n"
+                + "    \"current-snapshot-id\" : null,\n"
                 + "    \"refs\" : { },\n"
                 + "    \"snapshots\" : [ ],\n"
                 + "    \"statistics\" : [ ],\n"