If I conquer a battlefield with no enemy units present, is it considered as winning a combat?

No, conquering a battlefield with no enemy units present is not considered winning a combat.

According to Rule 438, a Combat only occurs when a battlefield has units controlled by two opposing players. If there are no enemy units present, a combat cannot be initiated or resolved.

Instead, when you move units to an empty battlefield (or one where you are the only player with units), you are performing a Showdown (or simply establishing control if no showdown is required).

Here is the breakdown based on the rules:

• Conquering: According to Rule 446.1, "Conquer" is defined as gaining control of a battlefield you did not yet score this turn. This can happen through combat resolution, but it also happens when you move units to an empty battlefield (Rule 8350 FAQ).
• Combat: According to Rule 430.1 and Rule 437, combat is a specific state that requires opposing units to be present. If you move to an empty battlefield, you are not in combat, so you cannot "win" a combat.

In summary: You successfully Conquer the battlefield, but because no combat took place, you did not win a combat.