FilePath: Return optional bytearray for file contents

For differentiating between "error" and "empty file".

Change-Id: I2c019ceac625e7be3180afa4d47ae3a24df91c1d
Reviewed-by: hjk <hjk@qt.io>

src/libs/utils/persistentsettings.cpp

@@ -185,7 +185,7 @@ private:
 
 QVariantMap ParseContext::parse(const FilePath &file)
 {
-    QXmlStreamReader r(file.fileContents());
+    QXmlStreamReader r(file.fileContents().value_or(QByteArray()));
 
     m_result.clear();
     m_currentVariableName.clear();