Fixed Energy.toString and mandatory parquet value omission

Before this change, a lower energy value was printed in KJoule and a
higher one in Joule. This change reverses it.

Before this change no exception was thrown when omitting value in
a required parquet field while exporting, resulting in unreadable
parquet file. Now exception is thrown while exporting.

opendc-common/src/main/kotlin/org/opendc/common/units/Energy.kt

@@ -55,7 +55,7 @@ public value class Energy private constructor(
     override fun toString(): String = fmtValue()
 
     override fun fmtValue(fmt: String): String =
-        if (value >= 1000.0) {
+        if (value <= 1000.0) {
             "${toJoule().fmt(fmt)} Joule"
         } else {
             "${toKJoule().fmt(fmt)} KJoule"

opendc-trace/opendc-trace-parquet/src/main/kotlin/org/opendc/trace/util/parquet/exporter/Exporter.kt

@@ -32,6 +32,7 @@ import org.apache.parquet.schema.PrimitiveType.PrimitiveTypeName.DOUBLE
 import org.apache.parquet.schema.PrimitiveType.PrimitiveTypeName.FLOAT
 import org.apache.parquet.schema.PrimitiveType.PrimitiveTypeName.INT32
 import org.apache.parquet.schema.PrimitiveType.PrimitiveTypeName.INT64
+import org.apache.parquet.schema.Type
 import org.apache.parquet.schema.Types
 import org.opendc.trace.util.parquet.ParquetDataWriter
 import java.io.File
@@ -122,7 +123,13 @@ public class Exporter<T : Exportable>
                                 val valueToAdd: Any =
                                     column.getValue(
                                         record,
-                                    ) ?: return@forEachIndexed // Maybe add explicit check for optional fields
+                                    ) ?: let {
+                                        if (column.field.isRepetition(Type.Repetition.OPTIONAL)) {
+                                            return@forEachIndexed
+                                        } else {
+                                            throw RuntimeException("trying to insert null value in required column $column")
+                                        }
+                                    }
 
                                 startField(column.name, idx)
                                 when (column.primitiveTypeName) {