FilePath: Return optional bytearray from device hooks

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

Change-Id: I1806bb7386f5e7db28489f9f7e685648bfc20110
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: hjk <hjk@qt.io>

src/libs/utils/filepath.cpp

@@ -534,7 +534,6 @@ std::optional<QByteArray> FilePath::fileContents(qint64 maxSize, qint64 offset)
 {
     if (needsDevice()) {
         QTC_ASSERT(s_deviceHooks.fileContents, return {});
-        // TODO change hooks to return optional byte array
         return s_deviceHooks.fileContents(*this, maxSize, offset);
     }