*Problem*

Gitaly::CommitDelta does not have a `#patch` attribute.
It leads to the problem that calls to methods `#too_large?`,
`#collapsed?`, `#has_binary_notice?` and `#to_hash` raise an
exception.

Here is an example of the exception:

```
Failure/Error:
@too_large = @diff.bytesize >= self.class.patch_hard_limit_bytes

NoMethodError:
 undefined method `bytesize' for nil:NilClass
```

*Solution*

I explicitly set `@diff` to have an empty string value if the provided
payload does not implement `#patch` method.