[core] Skip tag copy in fastForward when branch has no tags

[ArnavBalyan]

Purpose

• fastForward copies the tag directory to main, even when the branch did not have a tag created.
• On object store,listStatus on the missing directory throws FNF exception, causing failure for fastforward for branch without tags.
• Add a guard to check that the tag dir exists before the copy operation.
• Closes user reported bug [Bug] FileSystemBranchManager requires at least one tag on branch in fastForward #7781

Tests

• UT