fix: Make legacy property sturdier

This property might be accessed/called before we've properly initialized feature flags, so let's make sure we're not breaking anything (we have broken it already, see https://posthog.slack.com/archives/C02E3BKC78F/p1734111258352399)

src/posthog-core.ts

@@ -286,7 +286,7 @@ export class PostHog {
 
     // Legacy property to support existing usage - this isn't technically correct but it's what it has always been - a proxy for flags being loaded
     public get decideEndpointWasHit(): boolean {
-        return this.featureFlags.hasLoadedFlags
+        return this.featureFlags?.hasLoadedFlags ?? false
     }
 
     /** DEPRECATED: We keep this to support existing usage but now one should just call .setPersonProperties */