Can resource be considered scarce if its supply is limited, while quantity of the resource is practically unlimited? Let's consider energy of our Sun for an example. The Sun will be able to give us lots of energy for millions of years (and even its annual energy output is enormous), it's practically unlimited energy. We would have practically unlimited supply of solar energy if we built Dyson sphere. Yet there is limited amount of solar energy that we can collect, save and distribute due to our limited technologies, limited resources and other considerations (like we can't just replace all forests with solar panels, it will be huge blow to our ecology). We can't increase supply of solar energy to satisfy global total demand for energy.

Given this situation, can we say that in our current situation solar energy is a scarce resource? And if yes, then can we generalize this conclusion from this particular example to any case where we have limited supply of unlimited resource?

UPDATE: from comments:"you don’t need any other price than zero for this check, you need to check if at price 0 D>S". Now it seems to make sense.
Economists view everything people want, strive for, or can’t achieve effortlessly as scarce (you can read this nice article at EconLib). Under such broad definition virtually everything is scarce as anything requires at least some minuscule amounts of effort.

However, if you want it is possible to narrow this definition down and be more specific as in most models a scarcity can be checked and defined by looking whether at zero price there is still excess demand or not - if there is equilibrium then price must be positive, and we would say that this indicates scarcity.

Breathable atmosphere on land and outside extremely polluted city under such definition would not be considered scarce because supply is so high that at 0 price all demand is completely satisfied, but if you need some breathable atmosphere for scuba diving or in those cities where they sell oxygen tanks, or in space breathable atmosphere would be scarce because supply there is not large enough so that the equilibrium price is 0 and all demand is satisfied (there is no excess demand).

Now we can try to apply this to your example of Dyson sphere. First, I doubt this sphere will just pop out into existence and it will require no maintenance. There will be very likely exorbitant fixed and maintenance costs of such structure. Since the energy output will depend on how well the sphere is maintained I doubt it is even feasible to say equilibrium price of energy could ever be driven to zero as the supply depends on marginal costs which won’t be zero in that case. 

Second, let’s say that as a thought experiment that marginal costs of operating the sphere are 0, it would still not be enough. Sun gives out a lot of energy approximately (rounding up) $4 (10^{26})$ joules per second or wats. Now that is a large amount of energy but to build a Dyson sphere or Dyson Swarm (a more realistic version of the concept) you have to be at least type II civilization on Kardashev scale and such civilization would also have insatiable lust for more and more energy. Before I became economist I was studying astrophysics. We did not discussed Dyson sphere or swarm much but when it was brought up during break/after class discussions etc. it was always in context of being necessary to satisfy the needs of type II civilization which will probably want to run some ancestor simulations, or for building kugelblitz black holes that could serve as engines for starships or do other stuff that requires unfathomable amounts of energy. So even with output of whole start being collected such civilization would most likely still have such exorbitant demands for energy that it would still be expensive and price would be nowhere near zero, and there would be huge excess demand even at zero price. 

This being said assuming that such civilization would only need small fraction of the energy, and Dyson sphere/swarm would have no marginal costs, so everyone’s demand is satisfied (at zero price there is no excess demand) then you could argue energy ceases to be scarce. Also you can use the general definitions and this example to generalize this for any good/service.