This fix tries to address the issue raised in 29946 where
listing networks from API will return `null` if the result of
network filter is empty.
The reason for the issue was that inside the `filterNetworks()`,
the return value was initialized as `nil`:
```
var typeNet []types.NetworkResource
```
This is inconsistent with other places where return value was
initialized with `[]`
```
displayNet := []types.NetworkResource{}
```
This fix addresses the issue by changing `typeNet` to `[]` as well.
This fix fixes 29946.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>
Upstream-commit: 6ae709bf10e97c0a0ad8aebb9075f59eaea97ba9
Component: engine
6fcadc8e19