Samples: incorrect/misleading cached token lookup using `account=None`

[erharb]

In the 1-Call-MsGraph-WithSecret sample, the comments explain that calling app.acquire_token_silent will look up a token from cache (source).

# Firstly, looks up a token from cache
# Since we are looking for token for the current app, NOT for an end user,
# notice we give account parameter as None.
result = app.acquire_token_silent(config["scope"], account=None)

However, the current behavior of the acquire_token_silent function will immediately return None if the account parameter is provided as None as given in the sample (source).

    def acquire_token_silent(
            self,
            scopes,  # type: List[str]
            account,  # type: Optional[Account]
        ...
        if not account:
            return None  # A backward-compatible NO-OP to drop the account=None usage

Can the sample be updated to show how to properly retrieve a cached token?