Only call the ob_flush function if there is active buffer (#55141)

When ob_flush() is called without first calling ob_start(), it fails
saying there's no active buffer.

To avoid this issue, we must check the buffer level before flushing it.

Author: tonysm

src/Illuminate/Routing/ResponseFactory.php

@@ -150,7 +150,10 @@ public function eventStream(Closure $callback, array $headers = [], StreamedEven
                 echo 'data: '.$message;
                 echo "\n\n";
 
-                ob_flush();
+                if (ob_get_level() > 0) {
+                    ob_flush();
+                }
+
                 flush();
             }
 
@@ -166,7 +169,10 @@ public function eventStream(Closure $callback, array $headers = [], StreamedEven
                 echo 'data: '.$endStreamWith;
                 echo "\n\n";
 
-                ob_flush();
+                if (ob_get_level() > 0) {
+                    ob_flush();
+                }
+
                 flush();
             }
         }, 200, array_merge($headers, [