Don't assume `QStandardPaths::locate` succeeds

A QFile was being constructed directly from the result of
`QStandardPaths::locate`, which can return an empty string if it fails
to find a matching file. Instead we should explicitly check the path is
valid by checking if the string is empty - the result of
QFile("").exists() is not documented in the Qt documentation, even
though the implementation (at time of writing) will return false as
expected, so this is effectively fixing undefined behaviour.