resource/aws_rds_cluster: Prevent panic with missing master_password or master_username
resource/aws_rds_cluster: Prevent panic with missing master_password or master_username

Reference: https://github.com/terraform-providers/terraform-provider-aws/pull/7213#discussion_r259233400

Previous output from acceptance testing:

```
=== CONT  TestAccAWSRDSCluster_missingUserNameCausesError

------- Stderr: -------
panic: interface conversion: interface {} is nil, not string

goroutine 137 [running]:
github.com/terraform-providers/terraform-provider-aws/aws.resourceAwsRDSClusterCreate(0xc000349260, 0x3c28320, 0xc0001adc00, 0xc000349260, 0x0)
	/opt/teamcity-agent/work/2e10e023da0c7520/src/github.com/terraform-providers/terraform-provider-aws/aws/resource_aws_rds_cluster.go:718 +0x6d9d
```

Output from acceptance testing:

```
--- PASS: TestAccAWSRDSCluster_missingUserNameCausesError (3.24s)
```

terraform-providers/terraform-provider-aws,kjmkznr/terraform-provider-aws,Ninir/terraform-provider-aws,Ninir/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws,Ninir/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws,kjmkznr/terraform-provider-aws,Ninir/terraform-provider-aws