[bug] add a default of `seed: null` for the simulator config in the

schema

fixes the issue raised at !123 (comment 551445)

dnadna/schemas/simulation.yml

@@ -30,11 +30,12 @@ allOf:
                 default: 1
 
             seed:
-                type: "integer"
+                type: ["integer", "null"]
                 description: >-
                     fixed seed to use for seeding the random number
                     generator at the beginning of the simulation; if absent
                     then the PRNG's default seeding method is used
+                default: null
 
             summary_statistics: {"$ref": "summary-statistics.yml#/definitions/summary_statistics"}