Improve behavior when kfmclient openURL is launched with invalid URL

If kfmclient openURL is launched with an invalid URL, filteredURL will
return an empty URL which causes Konqueror to display an "Undocumented
error" error message.

To avoid this, when filteredUrl returns an empty URL, use the user's
home URL, as if no URL had been specified. In this case we also ignore
the mimetype requested by the user (if any) because in most cases it
won't be correct for the new URL.
2 jobs for !122 with avoid-undocumented-error-message in 7 minutes and 59 seconds (queued for 25 seconds)
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