Technically 0 out of 0 is undefined. However, the answer has been stated already.
To put it mathematic-ish-ly:
A system is represented by the following equation:
System=space + xplanets
To control space you need at least 1 non-fighter ship.
To control a planet you need a flag marker on it.
So control can of a system can be defined as:
A system can be controlled, if, and only if:
0=space+xplanets-space controlled-yplanets controlled
Where 'x' is the number of planets in the system and 'y' is the number of planets controlled.
The only time "0 out of 0" is undefined is when you are doing division. IE, 0 divided by 0 is undefined. But "0" itself doesn't always imply indefinition.
In your equation, if x is 0, and y is 0, the equation is defined just fine:
0 = 1 (space) +0 (planets) - 1 (space controlled) - 0 (planets controlled). You are not dividing by 0, so you are good to go.
If you were writing a program that checked control, you'd probably do something like this:
system_is_controlled = true;
if (! space_controlled) system_is_controlled = false;
for each planet in system_planets
if (! planet.controlled) system_is_controlled = false;
If there are no system planets, the loop would just be skipped. Thus, while in SOME cases, 0 out of 0 can be "undefined", it's not necessarily the case, it just depends on how you represent it.