fix: Mark project api key resource as destroyed if not present

[AgustinBettati]

Description

Ticket: INTMDB-918

Include missing logic to detect if project api key is not longer present during read operation.

Type of change:

• Bug fix (non-breaking change which fixes an issue). Please, add the "bug" label to the PR.
• New feature (non-breaking change which adds functionality). Please, add the "enhancement" label to the PR.
• Breaking change (fix or feature that would cause existing functionality to not work as expected). Please, add the "breaking change" label to the PR.
• This change requires a documentation update
• Documentation fix/enhancement

Required Checklist:

• I have signed the MongoDB CLA
• I have read the contribution guidelines
• I have added tests that prove my fix is effective or that my feature works per HashiCorp requirements
• I have added any necessary documentation (if appropriate)
• I have run make fmt and formatted my code

Further comments

• Included acceptance test to reproduce reported issue. Create project api key resource, delete externally using Atlas API, then expect terraform to detect resource needs to be recreated.

[andreaangiolillo]

LGTM