Using `/lagoon config add` with an empty `~/.lagoon.yml` file produces a panic error

Describe the bug
While using the uselagoon/lagoon-cli image in a gitlab CI job, we encounter the following error when trying to configure the cli:

panic: assignment to entry in nil map

This appears to happen all the way from v0.18.0 up until the current v0.21.1

To Reproduce

In the container, run

/lagoon config add --graphql $GRAPHQL_ENDPOINT --ui $UI_URL --hostname $SSH_HOST --lagoon $LAGOON --port 22 --force

(populating the shell variables with real things, obviously)

It results in:

panic: assignment to entry in nil map

goroutine 1 [running]:
github.com/uselagoon/lagoon-cli/cmd.glob..func7(0xc00015e700?, {0x9c1242?, 0x4?, 0x9c11ee?})
	/go/src/github.com/uselagoon/lagoon-cli/cmd/config.go:169 +0x249
github.com/spf13/cobra.(*Command).execute(0xde7840, {0xc00013a580, 0xb, 0xb})
	/go/pkg/mod/github.com/spf13/cobra@v1.8.0/command.go:983 +0xabc
github.com/spf13/cobra.(*Command).ExecuteC(0xdeba60)
	/go/pkg/mod/github.com/spf13/cobra@v1.8.0/command.go:1115 +0x3ff
github.com/spf13/cobra.(*Command).Execute(...)
	/go/pkg/mod/github.com/spf13/cobra@v1.8.0/command.go:1039
github.com/uselagoon/lagoon-cli/cmd.Execute()
	/go/src/github.com/uselagoon/lagoon-cli/cmd/root.go:113 +0x1a
main.main()
	/go/src/github.com/uselagoon/lagoon-cli/main.go:8 +0xf
+ /lagoon config default --lagoon XXXX
Chosen context 'XXXX' doesn't exist in config file55 <nil>
+ /lagoon version
lagoon v0.21.1 (go1.21.8)
built 2024-03-22T03:36:21+0000
+ /lagoon config list
NAME	VERSION	GRAPHQL	SSH-HOSTNAME	SSH-PORT	SSH-KEY
+ /lagoon login
Chosen context 'amazeeio' doesn't exist in config file
+ /lagoon whoami

Expected behavior
A config entry added to ~/.lagoon.yml

Additional context

As it turns out, the error is caused by the presence of a blank ~/.lagoon.yml in the container.

Simply removing the empty config file before attempting to add a config prevents the error, and is what we're currently using as a workaround.

Start in cmd/config.go at line 169, where the stack trace shows the panic during config add. Reproduce the command with an empty ~/.lagoon.yml, then verify that it completes without panicking and writes the requested configuration entry to the file.