Show notification, if API key is missing

Bence Szalairequested to merge
sbnc/code-stats-intellij:feat-show-notification-for-missing-api-key into master

Addresses #18.

Since I have already set up the project, and I looked around issues, I just thought it'd be a quick win, so here it is.

Interestingly the code contained checks for apiKey == null, but that (I guess) only happens the first time the plugin is installed. Later the user may delete the API key, in which case it'll become an empty string, so I've added apiKey.isEmpty() to the checks.

Edited by Bence Szalai

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