Avoid crash when metrics does not exists

[vmercierfr]

Objective

Avoid crash when metrics does not exists

Why

RDS exporter could crash when some EC2 information are empty.

It could occured for instances without EBS optimization or invalid state

How

• Handle empty nil pointer in EC2 package (AWS SDK built-in method)

Release plan

• [ ] Merge this PR
• [ ] Apply with CI