Fix join modifier when value is null (#3001)

src/Modifiers/CoreModifiers.php

@@ -753,6 +753,10 @@ public function image($value, $params)
      */
     public function joinplode($value, $params)
     {
+        if (is_null($value)) {
+            return '';
+        }
+
         // Workaround to support pipe characters. If there are multiple params
         // that means a pipe was used. We'll just join them for now.
         if (count($params) > 1) {

tests/Modifiers/JoinTest.php

@@ -0,0 +1,21 @@
+<?php
+
+namespace Tests\Modifiers;
+
+use Statamic\Modifiers\Modify;
+use Tests\TestCase;
+
+class JoinTest extends TestCase
+{
+    /** @test */
+    public function it_returns_empty_string_when_value_is_null()
+    {
+        $joined = $this->modify(null);
+        $this->assertEquals('', $joined);
+    }
+
+    public function modify($arr, $delimiter = ' ')
+    {
+        return Modify::value($arr)->join($delimiter)->fetch();
+    }
+}