Return false in case the url is empty.

src/ExternalLink.php

@@ -46,4 +46,74 @@ public function preProcess($data)
     {
         return preg_replace('/^(http|https):\/\//i', '', $data);
     }
+
+    <?php
+
+namespace Jonassiewertsen\ExternalLink;
+
+use Statamic\Fields\Fieldtype;
+
+class ExternalLink extends Fieldtype
+{
+    protected $icon = 'earth';
+
+    protected $categories = ['text'];
+
+    protected $regexPattern = '/^(http|https):\/\//i';
+
+    /**
+     * @return string
+     */
+    public static function title()
+    {
+        return 'External Link';
+    }
+
+    /**
+     * Adding the https:// before saving
+     *
+     * @param $data
+     *
+     * @return string
+     */
+    public function process($data)
+    {
+        if (! $data) {
+            return;
+        }
+
+        return 'https://' . $data;
+    }
+
+
+    /**
+     * Removing the https:// before displaying in field
+     *
+     * @param $data
+     *
+     * @return mixed
+     */
+    public function preProcess($data)
+    {
+        $data = preg_replace($this->regexPattern, '', $data);
+    }
+
+    /**
+     * Augment the value, which will return false in 
+     * case it only contains the https prefix.
+     *
+     * @param $value
+     *
+     * @return mixed
+     */
+    public function augment($value)
+    {
+        if ($value === 'https://') {
+            return false;
+        }
+
+        return $value;
+    }
+}
+
 }