Learn Hardcore Korean: sigani eobseumyeon yeokkkaji taeksireul tayo. - If I don’t have time, I take a taxi to the station.

시간이 uses the subject marker -이/가. Here, 시간(이) 없다 is an existence/possession-style expression meaning to not have time, and the thing that “doesn’t exist” is time, so 시간이 없다 is very common.

시간은 없으면 would add contrast/emphasis (as for time, if there isn’t any…), which can sound like you’re contrasting time with something else.
In casual speech, -이/가 is often dropped: 시간 없으면 역까지 택시 타요.

What does 없으면 mean grammatically?

없으면 = 없다 (to not have / there isn’t) + -(으)면 (if/when).

있으면 = if there is / if I have
없으면 = if there isn’t / if I don’t have

It sets up a condition: If there’s no time, …

How do I know whether -(으)면 means if or when?

-(으)면 can be either, depending on context:

if: uncertain/conditional situation (If I don’t have time, I take a taxi.)
when: more routine/whenever (When I don’t have time (i.e., whenever that happens), I take a taxi.)

With 타요 (present polite), this sentence often reads as a general habit: If/When I’m short on time, I take a taxi to the station.