feat!: make results column nullable

UPGRADE.md

@@ -46,3 +46,6 @@
 * `Snapshot::averageWaitTime()` now returns milliseconds instead of seconds.
 * `Snapshot::averageHandlingTime()` now returns milliseconds instead of seconds.
 * `Snapshot::averageProcessingTime()` now returns milliseconds instead of seconds.
+* The `results` column in the `processed_messages` table is now nullable. Create
+  a migration to capture this change. This is to allow for a future feature where
+  result processing/storage can be disabled.

config/doctrine/mapping/ProcessedMessage.orm.xml

@@ -18,6 +18,6 @@
         <field name="tags" column="tags" nullable="true" />
         <field name="failureType" column="failure_type" nullable="true" />
         <field name="failureMessage" column="failure_message" type="text" nullable="true" />
-        <field name="results" column="results" type="json" />
+        <field name="results" column="results" type="json" nullable="true" />
     </mapped-superclass>
 </doctrine-mapping>