Fix mongoc_uri_get_compressors docs

Author: kevinAlbs

src/libmongoc/doc/mongoc_uri_get_compressors.rst

@@ -19,7 +19,7 @@ Parameters
 Description
 -----------
 
-Returns a bson document with the enabled compressors as the keys if ``uri`` has compressors provided, otherwise ``NULL``.
+Returns a bson document with the enabled compressors as the keys or an empty document if no compressors were provided.
 
 Example
 -------

src/libmongoc/tests/test-mongoc-uri.c

@@ -1002,6 +1002,8 @@ test_mongoc_uri_compressors (void)
 
    uri = mongoc_uri_new ("mongodb://localhost/");
 
+   ASSERT (bson_empty (mongoc_uri_get_compressors (uri)));
+
 #ifdef MONGOC_ENABLE_COMPRESSION_SNAPPY
    capture_logs (true);
    mongoc_uri_set_compressors (uri, "snappy,unknown");