Don't build .kra file if image is empty

Happens when drag&dropping empty layers.

Can cause modulo by zero crash in `KisDocument::generatePreview`
when dragging onto canvas, or somewhere in QuaZip when dragging
outside of Krita.

BUG:429049

plugins/impex/kra/kra_converter.cpp

@@ -137,6 +137,10 @@ QVector<StoryboardComment> KraConverter::storyboardCommentList()
 
 KisImportExportErrorCode KraConverter::buildFile(QIODevice *io, const QString &filename)
 {
+    if (m_image->size().isEmpty()) {
+        return ImportExportCodes::Failure;
+    }
+
     setProgress(5);
     m_store = KoStore::createStore(io, KoStore::Write, m_doc->nativeFormatMimeType(), KoStore::Zip);