kdoctools_create_handbook/manpage: check INSTALL_DESTINATION for empty value
Passing an empty value is not specificed in the docs as special case, rather hints to someone using a bad argument, like a non-set dir variable.
E.g. seen with former-deprecated-now-unset var MAN_INSTALL_DIR.
Motivated by multimedia/dragon!26 (merged) & games/skladnik!9 (comment 903043)
Will break builds where hit (no other KF6 projects seen on lxr.kde.org besides dragonplayer), but in this case for good cause, as the installation is broken before by no longer happening.