Add empty state handling to updateAllMarketplaces

[Copilot]

Addresses review feedback on #4: updateAllMarketplaces() prints a summary message even when no GitHub marketplaces exist to update.

Changes

• Add early return in updateAllMarketplaces() when githubMarketplaces.length === 0
• Print "No marketplaces to update." matching pattern in listMarketplaces() and listEnabledPlugins()
• Add test case for empty GitHub marketplace scenario

// Before: prints "Updated 0 marketplace(s)" when only directory marketplaces exist
// After: prints "No marketplaces to update." and returns early
if (githubMarketplaces.length === 0) {
  console.log("No marketplaces to update.");
  return;
}

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