Whether the field has a double base or not, the running lane exists. Now, whether it's actually marked...heck, some fields you're lucky if the foul lines are drawn.
I don't know what your recourse is if the running lane is not drawn/marked, but it exists nonetheless.
Every field regardless of single or double base has a running lane. Yes, it's supposed to be marked but all lines on a field exist regardless of if they are there or not.
Just a side note. If one of the HCs decides to protest the game later because of a RLI you called when there is no lane marked, it would be disallowed on the face due to the fact that they started the game with full knowledge of the missing field markings and by doing so consented to the condition.