fix: Fix Unexpected token '?' error

A simple fix to resolve the Unexpected token '?' error when trying to
run the next project.

closes #14

lib/buildCSPHeaders.js

@@ -9,7 +9,7 @@ const devDirectives = {
 }
 
 function getCSPDirective(value, defaultValue) {
-	return [value ?? defaultValue].flat()
+	return [value || defaultValue].flat()
 }
 
 module.exports = function buildCSPHeaders(options = {}) {