Adjust KotlinJavaPsiFacade::knownClassNamesInPackage for empty scope

The method must return an empty set in that case, instead of null
that would mean we can't compute it.

The problem was found when running FirLoadCompiledKotlinGenerated

compiler/resolution.common.jvm/src/org/jetbrains/kotlin/resolve/jvm/KotlinJavaPsiFacade.java

@@ -52,10 +52,7 @@ import org.jetbrains.kotlin.name.ClassId;
 import org.jetbrains.kotlin.name.FqName;
 import org.jetbrains.kotlin.progress.ProgressIndicatorAndCompilationCanceledStatus;
 
-import java.util.ArrayList;
-import java.util.Arrays;
-import java.util.List;
-import java.util.Set;
+import java.util.*;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ConcurrentMap;
 
@@ -214,9 +211,12 @@ public class KotlinJavaPsiFacade implements Disposable {
         return javaClass;
     }
 
+    /**
+     * @return null in case the set of names is impossible to compute correctly
+     */
     @Nullable
     public Set<String> knownClassNamesInPackage(@NotNull FqName packageFqName, @NotNull GlobalSearchScope scope) {
-        if (scope == GlobalSearchScope.EMPTY_SCOPE) return null;
+        if (scope == GlobalSearchScope.EMPTY_SCOPE) return Collections.emptySet();
 
         KotlinPsiElementFinderWrapper[] finders = finders();