fix(extend-app-start): Guard setAppStartExtender against null

Match the NoOp-fallback convention used by other SentryOptions setters:
accept a @nullable argument and coalesce to NoOpAppStartExtender so the
@NotNull field and getter can never be nulled out.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: buenaflor

sentry/src/main/java/io/sentry/SentryOptions.java

@@ -2667,8 +2667,9 @@ public void setFullyDisplayedReporter(
   }
 
   @ApiStatus.Internal
-  public void setAppStartExtender(final @NotNull IAppStartExtender appStartExtender) {
-    this.appStartExtender = appStartExtender;
+  public void setAppStartExtender(final @Nullable IAppStartExtender appStartExtender) {
+    this.appStartExtender =
+        appStartExtender != null ? appStartExtender : NoOpAppStartExtender.getInstance();
   }
 
   /**