gui: PreviewGeneratorPlainText: Don't add header if we have nothing to show

gui/previewgeneratorplaintext.cpp

@@ -246,6 +246,8 @@ QString PreviewGeneratorPlainText::generateLineBasedPreviewText(QTextStream &in,
 			totalWordCountMap[it->first] = totalWordCountMap.value(it->first, 0) + it->second;
 		}
 	}
+	if(!resultText.isEmpty())
+	{
 		if(isTruncated)
 		{
 			header += "(truncated) ";
@@ -256,7 +258,9 @@ QString PreviewGeneratorPlainText::generateLineBasedPreviewText(QTextStream &in,
 		}
 		header += "<hr>";
 
-	return header + resultText;
+		resultText = header + resultText;
+	}
+	return resultText;
 }
 
 QSharedPointer<PreviewResult> PreviewGeneratorPlainText::generate(RenderConfig config, QString documentPath,