SqladminV1beta4 client returns an incorrect object if passed nil in get()

[Temikus]

Observe:
service is client instance here

service.get_instance(nil).to_h
=> #<Google::Apis::SqladminV1beta4::DatabaseInstance:0x00007ff731389c00 @kind="sql#instancesList">

if instance exist:

service.get_instance("real_instance").to_h
=> #<Google::Apis::SqladminV1beta4::DatabaseInstance:0x00007ff731490108
 @backend_type="SECOND_GEN",
...

if instance doesn't exist:

service.get_instance("not_real_instance").to_h
Google::Apis::ClientError: instanceDoesNotExist: The Cloud SQL instance does not exist.

EXPECTED RESULT:
Passing in nil should return either nil or an error.

ACTUAL RESULT:
Passing in nil returns an invalid empty object.

Somewhat related to #698, however, the behaviour is slightly different and API is different so submitting this separately. Feel free to dedup.

[bajajneha27]

Hi @Temikus,
Thank you for bringing this to our attention! It does seem like there might be an opportunity to have better validation on the service backend for empty input scenarios since client-side validations aren't our typical approach.
You'll have to raise a bug report directly with the SQLAdmin service team. They will be best positioned to investigate this issue and implement potential fixes.