fix(declarative) do not mis-detect mesh mode
fix(declarative) do not mis-detect mesh mode

When using declarative config and a `cluster_ca` is not declared,
mesh mode cannot be used. This means that `mesh_alpn` is not declared.

When performing a proxy call to the SSL port with wget like this:

```
wget https://localhost:8443/ --no-check-certificate
```

we get to a state where `getssl()` returns a value but `ssl:getAlpnSelected()`
returns `nil`. We need to avoid this check when `mesh_alpn` is not set.

(Interestingly, this happens with `wget` but does not happen with `curl -k`.)

Kong/kong,Kong/kong,Mashape/kong,Kong/kong