wrap: default values for netrc are empty string from python 3.11

From python 3.11 [1]:

> The entry in the netrc file no longer needs to contain all tokens.  The missing
> tokens' value default to an empty string.  All the tokens and their values now
> can contain arbitrary characters, like whitespace and non-ASCII characters.
> If the login name is anonymous, it won't trigger the security check.

[1] python/cpython@15409c7

mesonbuild/wrap/wrap.py

@@ -656,7 +656,7 @@ def get_netrc_credentials(self, netloc: str) -> T.Optional[T.Tuple[str, str]]:
             return None
 
         login, account, password = self.netrc.authenticators(netloc)
-        if account is not None:
+        if account:
             login = account
 
         return login, password