chore: move dest-dev-null to new non-dagger flow. (#59759)

Prep to transition dev-null onto the new flow set up in #59716 .

buildSrc/src/main/groovy/airbyte-connector-docker-convention.gradle

@@ -40,7 +40,8 @@ class AirbyteConnectorDockerConventionPlugin implements Plugin<Project> {
              *    The core plugin’s dockerCopyDockerfile task will copy
              *    it into build/airbyte/docker/Dockerfile automatically.
              * ------------------------------------------------------------ */
-            if (!dockerExt.dockerfile.present) {
+            def userFile = dockerExt.dockerfile.get().asFile
+            if (!userFile.exists()) {
                 dockerExt.dockerfile.set(project.rootProject.file(SHARED_DOCKERFILE))
             }