Issue #2613178 by mikeryan, heddn: default_value: null in static map skips empty rows

core/modules/migrate/src/Plugin/migrate/process/StaticMap.php

@@ -35,7 +35,7 @@ public function transform($value, MigrateExecutableInterface $migrate_executable
     }
     $new_value = NestedArray::getValue($this->configuration['map'], $new_value, $key_exists);
     if (!$key_exists) {
-      if (isset($this->configuration['default_value'])) {
+      if (array_key_exists('default_value', $this->configuration)) {
         if (!empty($this->configuration['bypass'])) {
           throw new MigrateException('Setting both default_value and bypass is invalid.');
         }

core/modules/migrate/tests/src/Unit/process/StaticMapTest.php

@@ -65,6 +65,17 @@ public function testMapWithInvalidSourceWithADefaultValue() {
     $this->assertSame($value, 'test');
   }
 
+  /**
+   * Tests when the source is invalid but there's a default value of NULL.
+   */
+  public function testMapWithInvalidSourceWithANullDefaultValue() {
+    $configuration['map']['foo']['bar'] = 'baz';
+    $configuration['default_value'] = NULL;
+    $this->plugin = new StaticMap($configuration, 'map', []);
+    $value = $this->plugin->transform(array('bar'), $this->migrateExecutable, $this->row, 'destinationproperty');
+    $this->assertNull($value);
+  }
+
   /**
    * Tests when the source is invalid and bypass is enabled.
    *