[query] support importing empty JSON objects

hail/src/main/scala/is/hail/expr/AnnotationImpex.scala

@@ -263,11 +263,15 @@ object JSONAnnotationImpex {
         if (t.size == 0)
           Annotation.empty
         else {
-          val annotationSize =
-            if (padNulls) t.size
-            else jfields.map { case (name, _) =>
+          val annotationSize = if (padNulls) {
+            t.size
+          } else if (jfields.size == 0) {
+            0
+          } else {
+            jfields.map { case (name, _) =>
               t.selfField(name).map(_.index).getOrElse(-1)
             }.max + 1
+          }
           val a = Array.fill[Any](annotationSize)(null)
 
           for ((name, jv2) <- jfields) {

hail/src/test/scala/is/hail/methods/ExprSuite.scala

@@ -6,12 +6,13 @@ import is.hail.check.Prop._
 import is.hail.check.Properties
 import is.hail.expr._
 import is.hail.expr.ir.IRParser
-import is.hail.types.virtual.{TInt32, Type}
+import is.hail.types.virtual._
 import is.hail.utils.StringEscapeUtils._
 
 import org.json4s._
 import org.json4s.jackson.JsonMethods._
 import org.testng.annotations.Test
+import org.apache.spark.sql.Row
 
 class ExprSuite extends HailSuite {
 
@@ -70,6 +71,28 @@ class ExprSuite extends HailSuite {
     p.check()
   }
 
+  @Test def testImportEmptyJSONObjectAsStruct(): Unit =
+    assert(JSONAnnotationImpex.importAnnotation(parse("{}"), TStruct()) == Row())
+
+  @Test def testExportEmptyJSONObjectAsStruct(): Unit =
+    assert(compact(render(JSONAnnotationImpex.exportAnnotation(Row(), TStruct()))) == "{}")
+
+  @Test def testRoundTripEmptyJSONObject(): Unit = {
+    val actual = JSONAnnotationImpex.exportAnnotation(
+      JSONAnnotationImpex.importAnnotation(parse("{}"), TStruct()),
+      TStruct(),
+    )
+    assert(compact(render(actual)) == "{}")
+  }
+
+  @Test def testRoundTripEmptyStruct(): Unit = {
+    val actual = JSONAnnotationImpex.importAnnotation(
+      JSONAnnotationImpex.exportAnnotation(Row(), TStruct()),
+      TStruct(),
+    )
+    assert(actual == Row())
+  }
+
   @Test def testImpexes(): Unit = {
 
     val g = for {