Skip follower databases with no changes in diff output

[alex-slynko]

When follower databases have no changes, the diff output previously printed empty headers like:

# Changes for follower database ghti-follower.eastus/hydro
# Changes for follower database copilotlimiter.eastus/hydro

This filters out follower diffs with empty change lists so they are omitted entirely from both the markdown and structured diff output.

[Copilot]

Pull request overview

This PR improves diff output readability by omitting follower database sections entirely when there are no follower changes, preventing empty headers from appearing in both markdown and structured diff results.

Changes:

• Filter FollowerDiffs to only those with Changes.Count > 0 when generating structured diff output.
• Apply the same filtering when generating markdown diff output (and associated logging).

Show a summary per file

File	Description
KustoSchemaTools/KustoSchemaHandler.cs	Skips follower diffs with empty change lists in both structured and markdown diff generation.

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• Files reviewed: 1/1 changed files
• Comments generated: 0