Support null body when invoking functions

Currently, when the body is null, the json.encode method will cast it to a string as part of the encoding. This results in the body containing `"null"` which can break some server-side json implementations.

Author: point-source

lib/src/functions_client.dart

@@ -41,7 +41,7 @@ class FunctionsClient {
     Map<String, dynamic>? body,
     ResponseType responseType = ResponseType.json,
   }) async {
-    final bodyStr = await compute(json.encode, body);
+    final bodyStr = body == null ? null : await compute(json.encode, body);
 
     final response = await (_httpClient?.post ?? http.post)(
       Uri.parse('$_url/$functionName'),