refactor: simplify headers loop by removing redundant nil check

[localnerve]

Summary

This PR removes a redundant if headers != nil check before iterating over the headers map in ExecuteGraphQL.

Rationale

In Go, the range keyword is designed to safely handle nil maps and slices by performing zero iterations. Explicitly checking for nil before a range loop is unnecessary and deviates from idiomatic Go practices.

Changes

• Cleaned up graphql.go by removing the outer conditional check for the headers parameter.
• No functional change; behavior remains identical for both nil and non-nil map inputs.