[APIv2] /networks endpoint returns null instead of [] when there're no networks

[riyad]

Is this a BUG REPORT or FEATURE REQUEST? (leave only one on its own line)

/kind bug

Description

Trying to find regressions by exercising the APIv2 trough docker-py's test suite (see #5386) I came across a flaky assertion in the TestNetworks::test_remove_network test.

It seems that Podman 3.0.0-dev wrongly returns null (instead of []) from the /networks endpoint when no networks are defined.

Steps to reproduce the issue:

1. make sure we have no networks

NAME    VERSION  PLUGINS

2. list the networks via API:

*   Trying /var/run/user/1000/podman/podman.sock:0...
* Connected to localhost (/run/user/1000/podman/podman.sock) port 80 (#0)
> GET /v1.40/networks HTTP/1.1
> Host: localhost
> User-Agent: curl/7.68.0
> Accept: */*
> 
* Mark bundle as not supporting multiuse
< HTTP/1.1 200 OK
< Api-Version: 1.40
< Content-Type: application/json
< Libpod-Api-Version: 2.0.0
< Server: Libpod/2.0.0 (linux)
< Date: Tue, 09 Feb 2021 20:30:40 GMT
< Content-Length: 5
< 
null
* Connection #0 to host localhost left intact

Describe the results you received:

Response was 200 null

Describe the results you expected:

Response was 200 []

Additional information you deem important (e.g. issue happens only occasionally):

FWIW: Podman 2.2.1 returns the correct result.

[rhatdan]

@jwhonce @baude @Luap99 PTAL

[Luap99]

I take this one.