Fix bug if we disable creation of security group

[nyf-nyf]

what

• Add logic to empty a predefined list if the security group doesn't exist

why

• Fix issue where disabling security group creation causes a terraform issue

references

• Closes https://github.com/cloudposse/terraform-aws-ec2-client-vpn/issues/37

[nyf-nyf]

The bug appeared, when we tried to disable the creation of security group and created it by ourselves. Then passed the id of the created security group to the input var "associated_security_group_ids". In this case the first argument in concat appears to be null.

[Zeskbest]

@nitrocode approve please, we dont want to create 13th fork =3

[nitrocode]

@nyf-nyf @Zeskbest please see comment

[Zeskbest]

@nitrocode Hi there! Do we need any more updates? =)

[nyf-nyf]

@nitrocode check please

[aknysh]

/test all