Fix "Link not found!" on foreign columns

It also generated invalid queries with no column name to select.

Signed-off-by: William Desportes <williamdes@wdes.fr>

libraries/classes/Display/Results.php

@@ -4416,7 +4416,10 @@ class Results
             $relation = $map[$meta->name];
             // Field to display from the foreign table?
             $dispval = '';
-            if ($relation[2] !== '') {
+
+            // Check that we have a valid column name
+            // Relation::getDisplayField() returns false by default
+            if ($relation[2] !== '' && $relation[2] !== false) {
                 $dispval = $this->getFromForeign($relation, $whereComparison);
             }