bug #47086 Workaround disabled "var_dump" (nicolas-grekas)

This PR was merged into the 4.4 branch.

Discussion
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Workaround disabled "var_dump"

| Q             | A
| ------------- | ---
| Branch?       | 4.4
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Tickets       | Fix #46881
| License       | MIT
| Doc PR        | -

Commits
-------

5e80717230 Workaround disabled "var_dump"

Author: nicolas-grekas

HttpClientTrait.php

@@ -106,7 +106,7 @@ private static function prepareRequest(?string $method, ?string $url, array $opt
         }
 
         // Validate on_progress
-        if (!\is_callable($onProgress = $options['on_progress'] ?? 'var_dump')) {
+        if (isset($options['on_progress']) && !\is_callable($onProgress = $options['on_progress'])) {
             throw new InvalidArgumentException(sprintf('Option "on_progress" must be callable, "%s" given.', \is_object($onProgress) ? \get_class($onProgress) : \gettype($onProgress)));
         }