[8.x] Fixed json validation check if there is encoded content

[tabuna]

Hey, in recent commits #38046 #38088, JSON validation has been added when the application/json header is present, but this is not always required as the content can be encoded. This PR adds a check that will only check the content if it hasn't been encoded.

[jessarcher]

Looks good 👍

I was going to wrap it in a try/catch to prevent it from ever returning a non status related message, but this works too.

[derekmd]

Is GZIP-encoded content causing the exception "Invalid JSON was returned from the route" to be thrown?

This change may not be needed with #38192 now submitted to fix #38178.

When building the new custom PHPUnit message in statusMessageWithDetails(), this error checking block (called via $this->json()) must always be skipped:

framework/src/Illuminate/Testing/TestResponse.php

Lines 851 to 857 in 7bd4e6d

	if (is_null($decodedResponse) || $decodedResponse === false) {
	if ($this->exception) {
	throw $this->exception;
	} else {
	PHPUnit::fail('Invalid JSON was returned from the route.');
	}
	}

Ideally calling TestResponse@assertStatus() should gracefully fallback to generic message "Expected response status code [{$expected}] but received {$actual}." when the JSON response 'errors' key isn't found. That "Invalid JSON was returned from the route" exception is meant for dev feedback when calling the assertJson*() methods on plain key/value payloads.