fix: update Jackson BOM to 2.20.2 and fix logstash expected YAML

Signed-off-by: Siqi Ding <dingdd@amazon.com>

Author: Davidding4718

data-prepper-api/src/test/resources/org/opensearch/dataprepper/model/configuration/sample_pipelines/sample_pipeline_plugin_empty_string.yaml

@@ -1,9 +1,9 @@
 test-pipeline:
   source:
-    stdin: ""
+    stdin: null
   processor:
-  - uppercase_string: null
+  - uppercase_string: ''
   sink:
-  - stdout: null
+  - stdout: ""
   workers: 1
   delay: 100

data-prepper-logstash-configuration/src/test/resources/org/opensearch/dataprepper/logstash/log-ingest-to-opensearch.expected.yaml

@@ -12,7 +12,7 @@ logstash-converted-pipeline:
         match:
           log:
             - "%{COMBINEDAPACHELOG}"
-    - drop_events: null
+    - drop_events: {}
     - key_value:
         source: "message"
         destination: "test"