Description:
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When I use str_replace on an associative array to replace '<br>' with "\n" I find that values that were null have been replaced with empty strings. This affects subsequent processing as an empty string is not the same as null.

Reproduce code:
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$pattern[]   = "<br>";
$pattern[]   = "<br/>";
$pattern[]   = "<br />";
$replacement = "\n";

$array = array('field1' => '<br>', 'field2' => '', 'field3' => null)
        
$array = str_replace($pattern, $replacement, $array);


Expected result:
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Null values should be left as null values, and not replaced with empty strings.

Actual result:
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Values in the array that were null are being replaced with empty strings.