fix: prevent access to null config_ in SimpleAbrManager (#5362)

Well sometimes network information change could be triggered even if
this.config_ is null, resulting in an exception thrown in the browser.
Doesn't affect playback, but affects the number of errors detected by
our tools 😅

Let's check if abrManager is enabled before checking its config

lib/abr/simple_abr_manager.js

@@ -54,7 +54,7 @@ shaka.abr.SimpleAbrManager = class {
     // of connectivity changes.
     if (navigator.connection && navigator.connection.addEventListener) {
       this.onNetworkInformationChange_ = () => {
-        if (this.config_.useNetworkInformation && this.enabled_) {
+        if (this.enabled_ && this.config_.useNetworkInformation) {
           this.bandwidthEstimator_ = new shaka.abr.EwmaBandwidthEstimator();
           if (this.config_) {
             this.bandwidthEstimator_.configure(this.config_.advanced);