Fix nil pointer dereference importing gitlab_user

[armsnyder]

Fixes #469

The issue was a missing nil check.

An importer test was ignoring the skip_confirmation field, probably masking the bug.

Note I replaced the importer with schema.ImportStatePassthrough because the importer was duplicated code from the read function, and the read function only uses the id attribute to read. The three extra attribute-sets (lines 31-33) are covered already by resourceGitlabUserSetToState.

[roidelapluie]

Thanks!