Simplify BigBlueButton::processJsonResponse, drop parameter

[donquixote]

That parameter is always '' as correctly pointed out by my IDE.
It's a private method, so...

(This is baseline for further changes)

[GhaziTriki]

payload is still used in sendRequest. Is it related to the draft PR injecting custom HTTP client?

[donquixote]

payload is still used in sendRequest. Is it related to the draft PR injecting custom HTTP client?

The point is: This is a private method, and in every call the $payload is empty string / default value.
So we can drop the parameter and always pass empty string / rely on the default for ->sendRequest().