[wmts] Check that we've got a file path before attempting to parse it

[gacarrillor]

Fix #51

Note: I've been unable to replicate the issue. QGIS has specific tests that verify that a filePath() is always set along with a QgsFetchedContent.ContentStatus.Finished status. See https://github.com/qgis/QGIS/blob/166f326b3324d555af80d1b1b3a9c23f028955b5/tests/src/python/test_qgsnetworkcontentfetcherregistry.py#L81

However, I've added a double check here (QGIS seems to do the same) to avoid attempting to parse the file if filePath() is empty, so we won't never get the error.