[MR AI Summary] Do not create a diff summary for empty diff

Background

In #408726 (comment 1696633587), we got a feedback wherein the merge request diff is empty. This is because an empty commit was pushed to MR source branch.

Internally, we still create an empty MergeRequestDiff if commit is empty.

Proposal

Check if MergeRequestDiff is empty before calling Llm::SummarizeMergeRequestService#execute in MergeRequestDiff#prepare_diff_summary.

We're also filtering out binary files from diffs when generating prompt which can lead to empty diff sent to AI. We should also not send a request to AI if this is the case.