title: How to check if value is defined in rego?
body: <p>I want to check if a variable is defined in OPA policy. </p>
  
  <pre><code>&gt; subject
  1 error occurred: 1:1: rego_unsafe_var_error: var subject is unsafe
  </code></pre>
  
  <p>Is there a function to check if a variable is defined</p>