Prevent event-watcher JSON/HCL replacements from truncating files

What happened:

The pipedv1 event watcher accepts jsonField and HCLField in an event replacement. When a matching event is handled, it does not modify that format; instead, it writes an empty byte slice to the configured file and commits the change. This truncates the JSON or HCL file to zero bytes.

commitFiles in pkg/app/pipedv1/eventwatcher/eventwatcher.go has empty jsonField and HCLField branches. After either branch, newContent is nil and upToDate is false, so the following os.WriteFile call overwrites the file with empty content.

What you expected to happen:

Either PipeCD should update only the configured JSON/HCL field with the event value, or it should reject the configuration/return an explicit unsupported-format error without changing the file. It must never truncate a repository file.

How to reproduce it:

1. Configure a pipedv1 Piped to watch the repository and create a repository file named version.json:

   {"image":"example/app:1.0.0","keep":"this value"}
   

2. Add an application configuration with an event watcher replacement using the documented jsonField option:

   apiVersion: pipecd.dev/v1beta1
   kind: Application
   spec:
     name: event-watcher-repro
     plugins:
       kubernetes: {}
     eventWatcher:
     - matcher:
         name: image-update
       handler:
         type: GIT_UPDATE
         config:
           replacements:
           - file: version.json
             jsonField: $.image
   

3. Register an image-update event with data example/app:2.0.0 and wait for the watcher to process it.

4. Inspect the commit created by the event watcher. version.json is empty (zero bytes), rather than containing the updated image value.

5. Repeat with an HCL file and HCLField; it follows the same empty branch and has the same truncation behavior.

Environment:

• piped version: reproduced from source at 3f7551c96 (please replace with the released image/version used for the report, if applicable)
• control-plane version: not version-specific; the faulty write is in the pipedv1 event-watcher code path
• Others: pipedv1 application configuration; Git update event watcher handler; JSON or HCL replacement target

Start in pkg/app/pipedv1/eventwatcher/eventwatcher.go at commitFiles and inspect the empty jsonField and HCLField branches together with the following os.WriteFile call. Reproduce the JSON and HCL configurations described in the issue, then verify that each replacement either updates its configured field or returns an explicit unsupported-format error without truncating the file.