[skip ci] PHRAS-3417_search-empty-fields_MASTER

add : special values "_empty_" and "_any_" to search empty field or not-empty field. Works only with = operator.
e.g.: Title=_empty_

wip/todo : unit test ; facet

lib/Alchemy/Phrasea/SearchEngine/Elastic/AST/KeyValue/MissingExpression.php

@@ -0,0 +1,49 @@
+<?php
+
+namespace Alchemy\Phrasea\SearchEngine\Elastic\AST\KeyValue;
+
+use Alchemy\Phrasea\SearchEngine\Elastic\AST\Node;
+use Alchemy\Phrasea\SearchEngine\Elastic\Search\QueryContext;
+use Alchemy\Phrasea\SearchEngine\Elastic\Search\QueryPostProcessor;
+
+class MissingExpression extends Node
+{
+    const MISSING_VALUE = '_empty_';
+
+    private $key;
+    private $value;
+
+    public function __construct(Key $key)
+    {
+        $this->key = $key;
+    }
+
+    public function buildQuery(QueryContext $context)
+    {
+        $query = [
+            'bool' => [
+                'must_not' => [
+                    'exists' => [
+                        'field' => $this->key->getIndexField($context, true)
+                    ]
+                ]
+            ]
+        ];
+
+        if ($this->key instanceof QueryPostProcessor) {
+            return $this->key->postProcessQuery($query, $context);
+        }
+
+        return $query;
+    }
+
+    public function getTermNodes()
+    {
+        return array();
+    }
+
+    public function __toString()
+    {
+        return sprintf('(<%s> == <%s>)', $this->key, self::MISSING_VALUE);
+    }
+}