Reset hash when an entity reference wasn't found

Work in progress patch. I want to add a test that tests importing terms and their parent as in the second example from the issue summary.

And here is a test with a file where referenced items can appear later in the file. It also fixes a bug introduced in the previous patch: in some cases when passing an empty value to the entity reference target, the hash of the feed item was reset as well. This bug was catched with the new test: for the term 'Europe' there is no parent defined in the CSV file, thus an empty value for the entity reference target.

Self-review:

1. +++ b/src/Feeds/Target/EntityReference.php
   @@ -102,6 +104,61 @@ class EntityReference extends FieldTargetBase implements ConfigurableTargetInter
   +    // Remove empty values first.
   +    $raw_values = array_filter($raw_values);
   +    foreach ($raw_values as $key => $value) {
   +      if (isset($value['target_id']) && strlen(trim($value['target_id'])) === 0) {
   +        unset($raw_values[$key]);
   +      }
   +    }
   +
   +    if (empty($raw_values)) {
   +      // No values given. Let the parent handle these.
   +      return parent::setTarget($feed, $entity, $field_name, $raw_values);
   +    }
   ...
   +    $values = $this->prepareValues($raw_values);
   +    if (count($values) !== count($raw_values)) {
   +      // Some of the given values were not found.
   +      $reimport = TRUE;
   +    }
   

   This looks a bit too complex. Filtering out empty values first implies that setTarget() exactly knows what prepareValues() does. If the logic of prepareValues() changes, this code could introduce a bug.

2. +++ b/src/Feeds/Target/EntityReference.php
   @@ -102,6 +104,61 @@ class EntityReference extends FieldTargetBase implements ConfigurableTargetInter
   +      foreach ($values as $subvalues) {
   +        if (array_key_exists('target_id', $subvalues) && is_null($subvalues['target_id'])) {
   +          $reimport = TRUE;
   +          break;
   +        }
   +      }
   

   This loop is not necessary if $reimport is already true.

3. +++ b/tests/src/Kernel/Feeds/Target/EntityReferenceTest.php
   @@ -0,0 +1,421 @@
   +   * When importing a feed with that references items that are imported by an
   

   "with that"

4. +++ b/tests/src/Kernel/Feeds/Target/EntityReferenceTest.php
   @@ -0,0 +1,421 @@
   +    // Create two feed types.
   

   Explain why two feed types need to be created.

5. +++ b/tests/src/Kernel/Feeds/Target/EntityReferenceTest.php
   @@ -0,0 +1,421 @@
   +   * Tests if articles get an assigned author later.
   

   "assigned" can be omitted.

6. +++ b/tests/src/Kernel/Feeds/Target/EntityReferenceTest.php
   @@ -0,0 +1,421 @@
   +    // Assert two created nodes.
   +    $this->assertNodeCount(3);
   

   'two' does not match '3'.

7. +++ b/tests/src/Kernel/Feeds/Target/EntityReferenceTest.php
   @@ -0,0 +1,421 @@
   +    // Assert that the node doesn't currently have an author.
   

   the first node.

8. +++ b/tests/src/Kernel/Feeds/Target/EntityReferenceTest.php
   @@ -0,0 +1,421 @@
   +    // And re-import first feed.
   

   "And re-import first feed. Previously imported articles now should get an author."

9. +++ b/tests/src/Kernel/Feeds/Target/EntityReferenceTest.php
   @@ -0,0 +1,421 @@
   +    // Reload node.
   

   "Reload node 1 and check if it got an author."

10. +++ b/tests/src/Kernel/Feeds/Target/EntityReferenceTest.php
    @@ -0,0 +1,421 @@
    +    // And re-import first feed again.
    

    "And re-import first feed again. No nodes should get updated."

11. +++ b/tests/src/Kernel/Feeds/Target/EntityReferenceTest.php
    @@ -0,0 +1,421 @@
    +    // Assert that 'Belgium' did not get a parent assigned, but the Netherlands
    

    Put "Netherlands" in quotes.

This is a simpler implementation that introduces the exception class "ReferenceNotFoundException". Hopefully this also works.
Also expanded the docs in the test to make things a bit more clear as noted in #4.

Fixing unit test EntityReferenceTest. Also make ReferenceNotFoundException extend EmptyFeedException for backwards compatibility. EntityReference::setTarget() is catching ReferenceNotFoundException first, so we should be good.

Committed #7.