When using smart router with static providers, FinalizedBlocksHashes is empty since static providers don't participate in Lava's finalization protocol. The cache code was attempting to unmarshal empty byte slices which fails with 'expected {} for map'.

This fix adds a length check before attempting to unmarshal, gracefully handling the case where FinalizedBlocksHashes is not populated.

Description

Closes: #XXXX

Author Checklist

All items are required. Please add a note to the item if the item is not applicable and
please add links to any relevant follow up issues.

I have...

• read the contribution guide
• included the correct type prefix in the PR title, you can find examples of the prefixes below:
• confirmed ! in the type prefix if API or client breaking change
• targeted the main branch
• provided a link to the relevant issue or specification
• reviewed "Files changed" and left comments if necessary
• included the necessary unit and integration tests
• updated the relevant documentation or specification, including comments for documenting Go code
• confirmed all CI checks have passed

Reviewers Checklist

All items are required. Please add a note if the item is not applicable and please add
your handle next to the items reviewed if you only reviewed selected items.

I have...

• confirmed the correct type prefix in the PR title
• confirmed all author checklist items have been addressed
• reviewed state machine logic, API design and naming, documentation is accurate, tests and test coverage