Fix: Allow empty kms_key_id to preserve backwards compatibility (AWS Launch Template)

[cschindlbeck]

Description

Recent commit #44505 forces to pass

key_kms_id = null

instead of empty string / no field.
This commit should allow empty strings for this field as before.

Relations

Addresses #44648

Discussion

Alternatively, one could do the logic upwards with something like this https://github.com/cattle-ops/terraform-aws-gitlab-runner/pull/1343/files

[github-actions]

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