Fix isSafari() throwing on React Native (fixes #7962) (#7963)

In #7929 some logic was added to Firestore to check for the browser being Safari. This new check for isSafari() unexpectedly threw an exception in React Native. This PR fixes the exception by explicitly checking for an object not being undefined before using it.

Fixes #7962

Author: Yonom

.changeset/purple-cooks-explode.md

@@ -0,0 +1,5 @@
+---
+'@firebase/util': patch
+---
+
+Fix isSafari() throwing on React Native

packages/util/src/environment.ts

@@ -138,6 +138,7 @@ export function isNodeSdk(): boolean {
 export function isSafari(): boolean {
   return (
     !isNode() &&
+    !!navigator.userAgent &&
     navigator.userAgent.includes('Safari') &&
     !navigator.userAgent.includes('Chrome')
   );