Porter returns null when listing installations

[erikced]

Describe the bug

Porter returns null instead of an empty list when outputting installations to json and there are no matching entries.

To Reproduce

Steps to reproduce the behavior:

1. Run porter installations list -o json -n some-nonexistent-namespace
2. Observe returned null

Expected behavior

The command returns an empty json list, [].

Version

Copy the output of porter version below
porter v1.2.1