`upload-charm` should catch and raise on empty credentials

The Kubeflow team had CI using this action recently fail when executed from a fork. We believe this is because the fork did not define the credentials secret, thus meaning the credentials argument passed to upload-charm was an empty string.

Assuming this is the correct failure mode, upload-charm should catch and fail with an obvious error when it is passed blank or obviously incorrect credentials. Calling the action with an empty credential is an especially easy error to make in github actions, as actions will silently use an uninstantiated variable rather than raise an error.