Fix underfined error in Gitlab::Git::Diff

*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.

changelogs/unreleased/fix_nil_class_for_bytesize_error.yml

+---
+title: Fix undefined error in Gitlab::Git::Diff
+merge_request: 32967
+author:
+type: fixed

lib/gitlab/git/diff.rb

@@ -225,7 +225,7 @@ module Gitlab
       end
 
       def init_from_gitaly(diff)
-        @diff = encode!(diff.patch) if diff.respond_to?(:patch)
+        @diff = diff.respond_to?(:patch) ? encode!(diff.patch) : ''
         @new_path = encode!(diff.to_path.dup)
         @old_path = encode!(diff.from_path.dup)
         @a_mode = diff.old_mode.to_s(8)

spec/lib/gitlab/git/diff_spec.rb

@@ -122,6 +122,36 @@ EOT
         end
       end
     end
+
+    context 'using a Gitaly::CommitDelta' do
+      let(:commit_delta) do
+        Gitaly::CommitDelta.new(
+          to_path: ".gitmodules",
+          from_path: ".gitmodules",
+          old_mode: 0100644,
+          new_mode: 0100644,
+          from_id: '357406f3075a57708d0163752905cc1576fceacc',
+          to_id: '8e5177d718c561d36efde08bad36b43687ee6bf0'
+        )
+      end
+      let(:diff) { described_class.new(commit_delta) }
+
+      it 'initializes the diff' do
+        expect(diff.to_hash).to eq(@raw_diff_hash.merge(diff: ''))
+      end
+
+      it 'is not too large' do
+        expect(diff).not_to be_too_large
+      end
+
+      it 'has an empty diff' do
+        expect(diff.diff).to be_empty
+      end
+
+      it 'is not a binary' do
+        expect(diff).not_to have_binary_notice
+      end
+    end
   end
 
   describe 'straight diffs' do