Fix some minor unmarshaling bugs

[jtraglia]

What type of PR is this?

Bug fix

What does this PR do? Why is it needed?

This PR fixes some bugs I noticed while reviewing the unmarshaling code for ExecutionPayload structs.

• If either excessBlobGas or blobGasUsed is set, ensure both are set.
• Update variable names to reflect the change from "data" to "blob".
• Replace instance of errors.Errorf with errors.New and remove punctuation.
• Check if withdrawal entry is nil before dereferencing it.
• Also check for nil transactions in one situation.
• Fix deneb test which didn't include withdrawals, so it was a bellatrix payload.
• Add new tests for some of the things I fixed.