I cast the vote for “flat” as an answer to the Poll Q; here’s why, and also why you may easily get misled.
First, the OP Q itself is entirely different, somewhat not applicable, and thus misleading. To make the observation of direct temperature effect, you would need to be playing the bass at a lower temperature. Significant temperature changes are more than about 10 C. With such changes, the strings would try to contract at a relative rate of roughly 12 ppm/C, while the wood in the neck and body would generally contract at a rate of only 3 - 5 ppm/C. The body and neck are generally bigger; so, they win, and the strings would be stretched. Here is a table of coefficients of thermal expansion (CTE) for various materials. Wood is anisotropic, being a natural composite, and has a low CTE parallel to the grain.
Coefficients of Linear Thermal Expansion
Fine. But, here are some points to consider.
First, we rarely play the instrument on a stage where the temperature was reduced by 10C (18F).That would be a stage temp of 50 F. I mean, I’ve done it. I remember playing an outdoors New Years Eve party in Michigan when it was 25 F one time. Not a good experience. But, thankfully that kind of gig is rare.
Perhaps a more typical exposure for the instrument to lower temperatures is in the belly of the plane, if you checked it while traveling. There, the temperature decrease would be somewhat like my NYE gig. Even then, we are talking about maybe a half or whole step increase in string tension. Not a whole lot.
Much more typically, the instrument is inside in a heated environment and does not get exposed to colder temps, except in transit. BUT, the other major player is humidity, which tends to bottom out when temps get cold. When that happens, the steel in the string has negligible response; but, the wood gets considerably smaller; though less so for truly older wood in vintage pieces. Also, roasted necks are less affected. When the humidity bottoms out, the strings respond by going flat. And, that response is far more noticeable than changes in temperature. Basses are great hygrometers.
So, with all that going on, the answer to the OP Q is “sharp;” but, actual experience, and the answer to the Poll Q will normally be “flat.” Isn’t nature wonderful?