fix --config not being validated in mirror-images command

[xrstf]

What this PR does / why we need it:
Since 2.21, the --config is optional for doing KKP upgrades, but it being nil can cause issues and I overlooked that the mirror-images command actually always needs a config. So this PR adds the missing validation for the CLI flag.

What type of PR is this?
/kind bug

Does this PR introduce a user-facing change? Then add your Release Note here:

Fix `--config` flag not being validated in `mirror-images` command in the KKP installer

Documentation:

NONE

[embik]

/retest

[ahmedwaleedmalik]

/approve

[kubermatic-bot]

LGTM label has been added.

Git tree hash: 09c81d166c7c94733d876ae8b8af224bf6f58d23

[kubermatic-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ahmedwaleedmalik, xrstf

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Needs approval from an approver in each of these files:

• cmd/kubermatic-installer/OWNERS [ahmedwaleedmalik,xrstf]

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[embik]

/cherry-pick release/v2.21

[kubermatic-bot]

@embik: #11146 failed to apply on top of branch "release/v2.21":

Applying: fix --config not being validated in mirror-images command
Using index info to reconstruct a base tree...
M	cmd/kubermatic-installer/cmd_mirror_images.go
Falling back to patching base and 3-way merge...
Auto-merging cmd/kubermatic-installer/cmd_mirror_images.go
CONFLICT (content): Merge conflict in cmd/kubermatic-installer/cmd_mirror_images.go
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
Patch failed at 0001 fix --config not being validated in mirror-images command
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To restore the original branch and stop patching, run "git am --abort".

In response to this:

/cherry-pick release/v2.21

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