BUG/MEDIUM: tcpcheck: Don't eval custom expect rule on an empty buffer

BUG/MEDIUM: tcpcheck: Don't eval custom expect rule on an empty buffer

The commit a664aa6a6 ("BUG/MINOR: tcpcheck: Be able to expect an empty
response") instroduced a regression for expect rules relying on a custom
function. Indeed, there is no check on the buffer to be sure it is not empty
before calling the custom function. But some of these functions expect to
have data and don't perform any test on the buffer emptiness.

So instead of fixing all custom functions, we just don't eval them if the
buffer is empty.

This patch must be backported but only if the commit above was backported
first.