Add an empty contributor file.

This is a workaround to make the CI action work as expected.
The CLA action failed in this PR:
#1

The error in the action log is:

```
Run contributor-assistant/[email protected]
CLA Assistant GitHub Action bot has started the process
Error: Could not retrieve repository contents: Not Found. Status: 404
Error: Cannot read properties of undefined (reading 'data')
```

See for example:
https://github.com/cowprotocol/flash-loan-wrapper-solver/actions/runs/13071434597/job/36473778174

This workaround was suggested in:
contributor-assistant/github-action#155 (comment)

signatures/version1/cla.json

@@ -0,0 +1,3 @@
+{
+  "signedContributors": []
+}