fix: return false from fetchField on empty result

SimplePdo::fetchRow() returns null when there are no rows. Guard
PdoWrapper::fetchField() so empty queries return false instead of
calling getData() on null. Closes #726.

flight/database/PdoWrapper.php

@@ -91,11 +91,14 @@ class PdoWrapper extends PDO
      * @param string $sql   - Ex: "SELECT id FROM table WHERE something = ?"
      * @param array<int|string,mixed> $params - Ex: [ $something ]
      *
-     * @return mixed
+     * @return mixed|false
      */
     public function fetchField(string $sql, array $params = [])
     {
         $result = $this->fetchRow($sql, $params);
+        if ($result === null || count($result) === 0) {
+            return false;
+        }
         $data = $result->getData();
         return reset($data);
     }

tests/PdoWrapperTest.php

@@ -71,6 +71,12 @@ class PdoWrapperTest extends TestCase
         $this->assertEquals(2, $id);
     }
 
+    public function testFetchFieldReturnsFalseWhenNoResults(): void
+    {
+        $id = $this->pdo_wrapper->fetchField('SELECT id FROM test WHERE id = ?', [999]);
+        $this->assertFalse($id);
+    }
+
     public function testFetchRow(): void
     {
         $row = $this->pdo_wrapper->fetchRow('SELECT * FROM test WHERE name = ?', ['two']);

tests/SimplePdoTest.php

@@ -468,4 +468,10 @@ class SimplePdoTest extends TestCase
         $id = $this->db->fetchField('SELECT id, name FROM users WHERE id = ?', [1]);
         $this->assertEquals(1, $id);
     }
+
+    public function testFetchFieldReturnsFalseWhenNoResults(): void
+    {
+        $value = $this->db->fetchField('SELECT name FROM users WHERE id = ?', [999]);
+        $this->assertFalse($value);
+    }
 }