Fix ContainsConstraints for array-valued custom field constraints in "*.search" methods

Summary: Fixes T11593. We ask for a list of values when searching for custom "link" fields, but don't handle it correctly when actually construcitng a query.

Test Plan:
Added this custom field:

```
{
  "mycompany.target-version": {
    "name": "Target Version",
    "type": "link",
    "search": true
  }
}
```

Set a task to "beta". Let daemons index it. Queried for:

```
constraints: {
  "custom.mycompany.target-version": [
    "beta"
  ]
}
```

Got just one result back.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11593

Differential Revision: https://secure.phabricator.com/D16508

src/infrastructure/customfield/standard/PhabricatorStandardCustomFieldLink.php

@@ -47,7 +47,12 @@ final class PhabricatorStandardCustomFieldLink
     PhabricatorCursorPagedPolicyAwareQuery $query,
     $value) {
 
-    if (strlen($value)) {
+    if (is_string($value) && !strlen($value)) {
+      return;
+    }
+
+    $value = (array)$value;
+    if ($value) {
       $query->withApplicationSearchContainsConstraint(
         $this->newStringIndex(null),
         $value);