[client] simplify url verification using a more Pythonic style

Author: Guillaume Dedrie

pycti/api/opencti_api_client.py

@@ -120,7 +120,7 @@ def __init__(
         self.ssl_verify = ssl_verify
         self.cert = cert
         self.proxies = proxies
-        if url is None or len(url) == 0:
+        if not url:
             raise ValueError("An URL must be set")
         if token is None or len(token) == 0 or token == "ChangeMe":
             raise ValueError("A TOKEN must be set")