Add null safety for SyncId in GithubSyncService

[Copilot]

The SyncId property is nullable but was used without null checks when creating dictionaries and working with HashSets, causing potential ArgumentNullException and NullReferenceException.

Changes

• Dictionary creation: Filter documents with null/empty SyncId before ToDictionary() to prevent null key exceptions
• Document processing loops: Skip documents with null/empty SyncId before HashSet operations
• Consistent validation: Use string.IsNullOrWhiteSpace() uniformly across all three null check locations

// Before: throws ArgumentNullException if any SyncId is null
var existingDocMap = existingDocuments.ToDictionary(d => d.SyncInfo.SyncId, d => d);

// After: gracefully filters out null/empty SyncIds
var existingDocMap = existingDocuments
    .Where(d => !string.IsNullOrWhiteSpace(d.SyncInfo.SyncId))
    .ToDictionary(d => d.SyncInfo.SyncId!, d => d);

Documents with null SyncId are now silently skipped during sync operations rather than causing runtime failures.

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