Check if `buildopts.get_rpm_whitelist()` is really set before using it.
    
    This fixes module builds which sets the `buildopts` for something else
    than the RPM whitelist.
    
    Before this commit, when `buildopts` was set, the RPM whitelist was
    always taken from the buildopts even if it was not defined there.
    This resulted in empty `rpm_whitelist` and therefore no pkglist set
    in Koji tag.
    
    In this commit, MBS uses whitelist from `buildopts` only if it is
    really set by the module maintainer.