Certainly there is light apart from that made by our sun. (The moon is actually a reflection, I'm sure you know.)
The point is that the plants and vegetation on planet earth needs light (energy) from our particular sun in order to be viable, does it not?
Of course God could make anything grow and survive with or without the sun.
But if He changes the laws of physics all the time at every whim, then it is useless talking about any historical or geological events because it could always be said that God simply changed the Physics of the Universe on that particular day, and thus all measurements we would take today would be distorted and useless.
Okay - Bible lesson.
In verse 3 of Genesis 1 - it says, "Then God said, 'let there be light', and there was light." [This light is the very essence of God - His zoe (meaning substance).]
However, in verse 14 of Genesis 1 - it says, "Then God said, 'let there be lights in the sky to separate day from night.'"
The LIGHT IN VERSE 3 - IS GOD - God produces the light.
The Bible is quite clear - there are 2 sets of lights - one is God - and the other is the sun and moon.