chore: make has_ai_task fields on workspace builds and template versions nullable (#18403)

The fields must be nullable because there’s a period of time between
inserting a row into the database and finishing the “plan” provisioner
job when the final value of the field is unknown.

coderd/wsbuilder/wsbuilder.go

@@ -427,7 +427,10 @@ func (b *Builder) buildTx(authFunc func(action policy.Action, object rbac.Object
 			},
 			// appease the exhaustruct linter
 			// TODO: set this to whether the build included a `coder_ai_task` tf resource
-			HasAITask: false,
+			HasAITask: sql.NullBool{
+				Bool:  false,
+				Valid: false,
+			},
 		})
 		if err != nil {
 			code := http.StatusInternalServerError