fixup! Fixed one failing test for #713: convertTo can create a tempor…

…ary non-nullable column with nulls in it. This is now avoided and the behavior has been simplified/stabilized

core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/impl/api/convertTo.kt

@@ -276,8 +276,8 @@ internal fun AnyFrame.convertToImpl(
 
     dsl.fillers.forEach { filler ->
         val paths = result.getColumnPaths(UnresolvedColumnsPolicy.Create, filler.columns).toSet()
-        missingPaths -= paths
         val (newPaths, existingPaths) = paths.partition { it in missingPaths }
+        missingPaths -= paths
 
         // first fill cols that are already in the df
         result = result.update { existingPaths.toColumnSet() }.with { filler.expr(this, this) }