Simplify some code with null coalesce operator

Author: javiereguiluz

Dumper/AbstractDumper.php

@@ -63,7 +63,7 @@ public function __construct($output = null, string $charset = null, int $flags =
      */
     public function setOutput($output)
     {
-        $prev = null !== $this->outputStream ? $this->outputStream : $this->lineDumper;
+        $prev = $this->outputStream ?? $this->lineDumper;
 
         if (\is_callable($output)) {
             $this->outputStream = null;

Dumper/HtmlDumper.php

@@ -153,7 +153,7 @@ public function dump(Data $data, $output = null, array $extraDisplayOptions = []
      */
     protected function getDumpHeader()
     {
-        $this->headerIsDumped = null !== $this->outputStream ? $this->outputStream : $this->lineDumper;
+        $this->headerIsDumped = $this->outputStream ?? $this->lineDumper;
 
         if (null !== $this->dumpHeader) {
             return $this->dumpHeader;
@@ -964,7 +964,7 @@ protected function dumpLine($depth, $endOfValue = false)
         if (-1 === $this->lastDepth) {
             $this->line = sprintf($this->dumpPrefix, $this->dumpId, $this->indentPad).$this->line;
         }
-        if ($this->headerIsDumped !== (null !== $this->outputStream ? $this->outputStream : $this->lineDumper)) {
+        if ($this->headerIsDumped !== ($this->outputStream ?? $this->lineDumper)) {
             $this->line = $this->getDumpHeader().$this->line;
         }