Add mandatory capabilities parameter in the body for the webdriver new session command (#17212)

diff --git a/tools/webdriver/webdriver/client.py b/tools/webdriver/webdriver/client.py
index 52ebc95..1ce4906 100644
--- a/tools/webdriver/webdriver/client.py
+++ b/tools/webdriver/webdriver/client.py
@@ -413,7 +413,7 @@
         if self.session_id is not None:
             return
 
-        body = {}
+        body = {"capabilities": {}}
 
         if self.requested_capabilities is not None:
             body["capabilities"] = self.requested_capabilities