Use template for _buildNullDisplay method

Signed-off-by: Maurício Meneghini Fauth <mauriciofauth@gmail.com>

libraries/classes/Display/Results.php

@@ -2362,10 +2362,10 @@ class Results
     /**
      * Prepares the display for a null value
      *
-     * @param string $class           class of table cell
-     * @param bool   $condition_field whether to add CSS class condition
-     * @param object $meta            the meta-information about this field
-     * @param string $align           cell alignment
+     * @param string $class          class of table cell
+     * @param bool   $conditionField whether to add CSS class condition
+     * @param object $meta           the meta-information about this field
+     * @param string $align          cell alignment
      *
      * @return string  the td
      *
@@ -2375,18 +2375,16 @@ class Results
      *          _getDataCellForGeometryColumns(),
      *          _getDataCellForNonNumericColumns()
      */
-    private function _buildNullDisplay($class, $condition_field, $meta, $align = '')
+    private function _buildNullDisplay($class, $conditionField, $meta, $align = '')
     {
-        // the null class is needed for grid editing
-        $decimals = isset($meta->decimals) ? $meta->decimals : '-1';
-        return '<td ' . $align . ' data-decimals="' . $decimals
-            . '" data-type="' . $meta->type . '"  class="'
-            . $this->_addClass(
-                $class, $condition_field, $meta, ''
-            )
-            . ' null"><i>NULL</i></td>';
-    } // end of the '_buildNullDisplay()' function
+        $classes = $this->_addClass($class, $conditionField, $meta, '');
 
+        return Template::get('display/results/null_display')->render([
+            'align' => $align,
+            'meta' => $meta,
+            'classes' => $classes,
+        ]);
+    }
 
     /**
      * Prepares the display for an empty value

templates/display/results/null_display.twig

@@ -0,0 +1,7 @@
+<td {{ align }}
+    data-decimals="{{ meta.decimals is defined ? meta.decimals : '-1' }}"
+    data-type="{{ meta.type }}"
+    {# The null class is needed for grid editing #}
+    class="{{ classes }} null">
+    <em>NULL</em>
+</td>

test/classes/Display/ResultsTest.php

@@ -1533,8 +1533,12 @@ class ResultsTest extends PmaTestCase
                 array(),
                 0,
                 0,
-                '<td  data-decimals="0" data-type="string"  '
-                . 'class="grid_edit  null"><i>NULL</i></td>'
+                '<td ' . PHP_EOL
+                . '    data-decimals="0"' . PHP_EOL
+                . '    data-type="string"' . PHP_EOL
+                . '        class="grid_edit  null">' . PHP_EOL
+                . '    <em>NULL</em>' . PHP_EOL
+                . '</td>' . PHP_EOL
             ),
             array(
                 'all',