I think the key is the destruction of the star spreads it's matter out all over the star system. The gravitational forces are no longer at the point of the star.
In the dialogue between Data and Picard:
PICARD: It's close to Veridian Three ...but not close enough. ...Data, what would happen to the ribbon's course if Soran destroyed the Veridian star itself? ...That's where he's going.
DATA: It should be noted, sir, that the collapse of the Veridian star would produce a shock wave similar to the one we observed at Amargosa.
PICARD: Destroying all the planets in this system. - Generations transcript
The Enterprise moves to avoid the shock wave from the destruction of the Amargosa star.
Slightly before in the above conversation.
DATA: The destruction of the Amargosa star has altered the gravitational forces throughout this sector. As a result any ship passing through this region would have to make a minor course correction.
He says it is a minor course change, so it wasn't like everything in the universe was affected by this redistribution of gravity, but it was enough that things within the system were affected (ships need to change course, planets were destroyed, etc).
Sounds to me like the gravity would be equalized across the system, instead of being focused at the center, where it could affect the path of the Nexus.
For reference I've added the shots of the Nexus' course being changed by the absence of the Veridian star.
All pictures sourced from http://movie-screencaps.com/star-trek-generations-1994/29/