title: TCPDF reports "Undefined index: style" on single <hr> tag
body: <p>A simple</p>
  <pre><code>$pdf-&gt;writeHTML(&quot;&lt;hr&gt;&quot;, true, false, false, false, '');
  </code></pre>
  <p>which did work when last tested (a few versions of TCPDF ago), now results in &quot;Undefined index: style&quot;.</p>
  <p>The relevant code from the TCPDF sources is</p>
  <pre><code>case 'hr': {
      if ((isset($tag['height'])) AND ($tag['height'] != '')) {
          $hrHeight = $this-&gt;getHTMLUnitToUnits($tag['height'], 1, 'px');
      } else {
          $hrHeight = $this-&gt;GetLineWidth();
      }
      $this-&gt;addHTMLVertSpace($hbz, max($hb, ($hrHeight / 2)), $cell, $firsttag);
      $x = $this-&gt;GetX();
      $y = $this-&gt;GetY();
      $wtmp = $this-&gt;w - $this-&gt;lMargin - $this-&gt;rMargin;
      if ($cell) {
          $wtmp -= ($this-&gt;cell_padding['L'] + $this-&gt;cell_padding['R']);
      }
      if ((isset($tag['width'])) AND ($tag['width'] != '')) {
          $hrWidth = $this-&gt;getHTMLUnitToUnits($tag['width'], $wtmp, 'px');
      } else {
          $hrWidth = $wtmp;
      }
      $prevlinewidth = $this-&gt;GetLineWidth();
  
      $this-&gt;SetLineWidth($hrHeight);
          $lineStyle = array(
          'color' =&gt; $tag['fgcolor'],
          'cap'   =&gt; $tag['style']['cap'], // Error thrown here.
          'join'  =&gt; $tag['style']['join'],
          'dash'  =&gt; $tag['style']['dash'],
          'phase' =&gt; $tag['style']['phase'],
      );
  
      $lineStyle = array_filter($lineStyle);
  
      $this-&gt;Line($x, $y, $x + $hrWidth, $y, $lineStyle);
      $this-&gt;SetLineWidth($prevlinewidth);
      $this-&gt;addHTMLVertSpace(max($hbc, ($hrHeight / 2)), 0, $cell, !isset($dom[($key + 1)]));
      break;
  </code></pre>
  <p>The value of <code>$tag</code> is</p>
  <pre><code>Array
  (
      [elkey] =&gt; 0
      [tag] =&gt; 1
      [value] =&gt; hr
      [block] =&gt; 1
      [opening] =&gt; 1
      [parent] =&gt; 0
      [self] =&gt; 1
      [hide] =&gt; 
      [fontname] =&gt; dejavusans
      [fontstyle] =&gt; 
      [fontsize] =&gt; 10
      [font-stretch] =&gt; 100
      [letter-spacing] =&gt; 0
      [stroke] =&gt; 0
      [fill] =&gt; 1
      [clip] =&gt; 
      [line-height] =&gt; 1.25
      [bgcolor] =&gt; 
      [fgcolor] =&gt; Array
          (
              [R] =&gt; 0
              [G] =&gt; 0
              [B] =&gt; 0
          )
  
      [strokecolor] =&gt; Array
          (
              [R] =&gt; 0
              [G] =&gt; 0
              [B] =&gt; 0
          )
  
      [align] =&gt; 
      [listtype] =&gt; 
      [text-indent] =&gt; 0
      [text-transform] =&gt; 
      [border] =&gt; Array
          (
          )
  
      [dir] =&gt; ltr
      [attribute] =&gt; Array
          (
          )
  
  )
  </code></pre>
  <p>As can be seen, there is no <code>style</code> key. Am I doing something wrong, or is this a bug in TCPDF? How can I work around it?</p>