MimeUtils: fix warnings for missing mime types

Previously when opening the open dialog you'd have the warnings:
kf.kio.filewidgets.kfilefiltercombo: "" is not a valid mimeType
kf.kio.filewidgets.kfilefiltercombo: "image/x-nikon-nrw" is not a valid mimeType
kf.kio.filewidgets.kfilefiltercombo: "image/x-samsung-srw" is not a valid mimeType
kf.kio.filewidgets.kfilefiltercombo: "" is not a valid mimeType
kf.kio.filewidgets.kfilefiltercombo: "image/x-nikon-nrw" is not a valid mimeType
kf.kio.filewidgets.kfilefiltercombo: "image/x-samsung-srw" is not a valid mimeType

After this patch you have :
org.kde.kdegraphics.gwenview.lib: Unresolved mime type  "image/x-mng"
org.kde.kdegraphics.gwenview.lib: Unresolved raw mime type  "image/x-nikon-nrw"
org.kde.kdegraphics.gwenview.lib: Unresolved raw mime type  "image/x-samsung-srw"