When a template parameter is accidentially unindented, the stack
attributes become empty. This resulted in:

```
lib/stack_master/config.rb:123:in `block (2 levels) in load_stacks':
undefined method `[]' for nil:NilClass (NoMethodError)

          stack_attributes['allowed_accounts'] = attributes['allowed_accounts'] if attributes['allowed_accounts']
```

Now it will report a ConfigParseError with a more specific message.

Co-authored-by: Orien Madgwick <[email protected]>