mprisplugin: Don't skip if size is invalid and take at least one image
The `isValid()` check was supposed to take the first artwork it finds but a default-constructed `QSize` has zero size, which is a valid size. Moreover, if we failed to parse the size, we won't be bigger than a valid size it found before, so the `continue` is redundant and would cause it not to take the first artwork it found.
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