allow null for log format in stack spec (#562)

* allow null for log format in stack spec

* build stack spec

---------

Co-authored-by: Alexander Mattoni <5110855+mattoni@users.noreply.github.com>

Author: blewisCycle

stackspec/schema/StackSpecContainerConfigIntegrations.yml

@@ -226,6 +226,7 @@ properties:
                         enum:
                           - ndjson-headers
                           - ndjson-raw
+                      - type: "null"
                       - $ref: StackVariable.yml
               - type: "null"
               - $ref: StackVariable.yml