Improved error handling for negative lap times

ref #71

src/models/entities/Track.ts

@@ -43,10 +43,9 @@ export class Track {
    * The minimum time a runner should take to run a lap on this track.
    * Will be used for fraud detection.
    */
-  @Column()
+  @Column({ nullable: true })
   @IsInt()
   @IsOptional()
-  @IsPositive()
   minimumLapTime?: number;
 
   /**