opengl: correct detection of virtual packages having no dependencies

I believe the original intent was to make it that the configure step
for the opengl virtual packages fails if there is not at least one
dependency. This patch fixes the logic so that it actually fails if
dependency list is empty

Signed-off-by: Will Wagner <will_wagner@carallon.com>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>