initialize Google Analytics only if GA_ID is set

src/lib/analytics.js

 import GoogleAnalytics from 'react-ga';
 
-GoogleAnalytics.initialize(process.env.GA_ID || window.GA_ID, {
-    debug: (process.env.NODE_ENV !== 'production'),
-    titleCase: true,
-    sampleRate: (process.env.NODE_ENV === 'production') ? 100 : 0,
-    forceSSL: true
-});
+const GA_ID = (process.env.GA_ID || window.GA_ID);
+if (GA_ID) {
+    GoogleAnalytics.initialize(GA_ID, {
+        debug: (process.env.NODE_ENV !== 'production'),
+        titleCase: true,
+        sampleRate: (process.env.NODE_ENV === 'production') ? 100 : 0,
+        forceSSL: true
+    });
+}
 
 export default GoogleAnalytics;