fix: Get rid of the <nil> fields in all log outputs

[mkilchhofer]

Fixes #715

Before change (status quo)

env NEBRASKA_LOG_FORMAT unset

<nil> DBG Unknown format context=api logFormat=
<nil> DBG Unknown format context=nebraska logFormat=
<nil> DBG Unknown format context=auth logFormat=
<nil> DBG Unknown format context=omaha logFormat=
<nil> DBG Unknown format context=nebraska logFormat=
<nil> DBG Unknown format context=nebraska logFormat=
<nil> DBG Unknown format context=syncer logFormat=

   ____    __
  / __/___/ /  ___
 / _// __/ _ \/ _ \
/___/\__/_//_/\___/ v4.10.2
High performance, minimalist Go web framework
https://echo.labstack.com
____________________________________O/_______
                                    O\
⇨ http server started on [::]:8000

env NEBRASKA_LOG_FORMAT=pretty

<nil> DBG Unknown format context=api logFormat=pretty
<nil> DBG Unknown format context=nebraska logFormat=pretty
<nil> DBG Unknown format context=auth logFormat=pretty
<nil> DBG Unknown format context=omaha logFormat=pretty
<nil> DBG Unknown format context=nebraska logFormat=pretty
<nil> DBG Unknown format context=nebraska logFormat=pretty
<nil> DBG Unknown format context=syncer logFormat=pretty

   ____    __
  / __/___/ /  ___
 / _// __/ _ \/ _ \
/___/\__/_//_/\___/ v4.10.2
High performance, minimalist Go web framework
https://echo.labstack.com
____________________________________O/_______
                                    O\
⇨ http server started on [::]:8000

env NEBRASKA_LOG_FORMAT=json

   ____    __
  / __/___/ /  ___
 / _// __/ _ \/ _ \
/___/\__/_//_/\___/ v4.10.2
High performance, minimalist Go web framework
https://echo.labstack.com
____________________________________O/_______
                                    O\
⇨ http server started on [::]:8000
{"level":"info","machineId":"aaaa671d61774703ac7be60715220baa","appID":"e96281a6-d1af-4bde-9a0a-97b76e56dc57","group":"5b810680-e36a-4879-b98a-4f989e80b899","event":"update complete.success","previousVersion":"","context":"omaha","message":"processEvent eventError 0"}

After this change

env NEBRASKA_LOG_FORMAT unset

2024-01-21T10:57:30+01:00 DBG Unknown format context=api logFormat=
2024-01-21T10:57:30+01:00 DBG Unknown format context=nebraska logFormat=
2024-01-21T10:57:30+01:00 DBG Unknown format context=auth logFormat=
2024-01-21T10:57:30+01:00 DBG Unknown format context=omaha logFormat=
2024-01-21T10:57:30+01:00 DBG Unknown format context=nebraska logFormat=
2024-01-21T10:57:30+01:00 DBG Unknown format context=nebraska logFormat=
2024-01-21T10:57:30+01:00 DBG Unknown format context=syncer logFormat=

   ____    __
  / __/___/ /  ___
 / _// __/ _ \/ _ \
/___/\__/_//_/\___/ v4.10.2
High performance, minimalist Go web framework
https://echo.labstack.com
____________________________________O/_______
                                    O\
⇨ http server started on [::]:8000

env NEBRASKA_LOG_FORMAT=pretty

2024-01-21T11:17:25+01:00 DBG Unknown format context=api logFormat=pretty
2024-01-21T11:17:25+01:00 DBG Unknown format context=nebraska logFormat=pretty
2024-01-21T11:17:25+01:00 DBG Unknown format context=auth logFormat=pretty
2024-01-21T11:17:25+01:00 DBG Unknown format context=omaha logFormat=pretty
2024-01-21T11:17:25+01:00 DBG Unknown format context=nebraska logFormat=pretty
2024-01-21T11:17:25+01:00 DBG Unknown format context=nebraska logFormat=pretty
2024-01-21T11:17:25+01:00 DBG Unknown format context=syncer logFormat=pretty

   ____    __
  / __/___/ /  ___
 / _// __/ _ \/ _ \
/___/\__/_//_/\___/ v4.10.2
High performance, minimalist Go web framework
https://echo.labstack.com
____________________________________O/_______
                                    O\
⇨ http server started on [::]:8000
2024-01-21T11:17:27+01:00 INF processEvent eventError 0 appID=e96281a6-d1af-4bde-9a0a-97b76e56dc57 context=omaha event="update complete.success" group=5b810680-e36a-4879-b98a-4f989e80b899 machineId=aaaa671d61774703ac7be60715220baa previousVersion=

env NEBRASKA_LOG_FORMAT=json

   ____    __
  / __/___/ /  ___
 / _// __/ _ \/ _ \
/___/\__/_//_/\___/ v4.10.2
High performance, minimalist Go web framework
https://echo.labstack.com
____________________________________O/_______
                                    O\
⇨ http server started on [::]:8000
{"level":"info","machineId":"aaaa671d61774703ac7be60715220baa","appID":"e96281a6-d1af-4bde-9a0a-97b76e56dc57","group":"5b810680-e36a-4879-b98a-4f989e80b899","event":"update complete.success","previousVersion":"","time":"2024-01-21T10:58:28+01:00","context":"omaha","message":"processEvent eventError 0"}

[dongsupark]

Looks good. Tested locally, and it works well.
Thank you for your contribution!