Handle empty parameters in POST call better (#42)

PHP is encoding the empty parameters as an empty JSON list rather than the empty JSON object we need.  Explicitly test for empty parameters.

Author: AaronAtDuo

src/Client.php

@@ -214,7 +214,7 @@ public function apiCall($method, $path, $params, $additional_headers = [])
         $headers = [];
         if (in_array($method, ["POST", "PUT", "PATCH"], true)) {
             ksort($params);
-            $body = json_encode($params);
+            $body = empty($params) ? "{}" : json_encode($params);
             $params = [];
             $headers["Content-Type"] = "application/json";
             $uri = $path;