The relevant law is Texas Administrative Code Rule §81.35 "Voting from Outer Space" and who's going to vote against that? It is a very small modification of Texas early voting law.
(a) A person who meets the eligibility requirements of a voter under the Texas Election Code, Chapter 101, but who will be on a space flight during the early-voting ...
As explained in video description of https://www.youtube.com/watch?v=9Emsf_udovU the voting is done electronically
Most U.S. astronauts live in Houston. Texas law allows them to vote from space using a secure electronic ballot. Mission Control forwards the ballot to the space station and relays the completed ballot back to the county clerk.
Partial answer, exploring some details
It's clearly not an artefact of the SVG vectorised file on Wikipedia. A photograph of the Voyager Golden record (which features the same map), shows the same gap.
Using this to decode the pulsar map, the distance of the most similar pulsar can be established (280 parsecs)
The gap is then around ~200 parsecs. I ...