Error "CORS Rejected - Invalid origin" on RPC calls

[ahamlat]

Description

With origin header is set to empty, meaning that the header exists but doesn't have a value, we have this error : "CORS Rejected - Invalid origin" when calling RPC methods.
This is related to this #6666, as the regex is now more restrictive.

Acceptance Criteria

RPC methods should reply with the correct output when RPC and the namespace is enabled.

Steps to Reproduce (Bug)

1. Start Besu with RPC and ETH namespace enabled
2. Call eth_blockNumber

curl --location 'http://localhost:8545' \
--header 'Origin;' \
--header 'Content-Type: application/json' \
--data '{
    "jsonrpc": "2.0",
    "method": "eth_blockNumber",
    "params": [],
    "id": 1
}'

3. Besu replies with Exception thrown handling RPC and we can see this error line in the code

{"@timestamp":"2024-04-23T13:24:05,906","level":"ERROR","thread":"vert.x-eventloop-thread-1","class":"Logging403ErrorHandler","message":"CORS Rejected - Invalid origin","throwable":""}

Expected behavior: [What you expect to happen]
Besu should reply here with the chain head block number

Actual behavior: [What actually happens]
Error

Frequency: [What percentage of the time does it occur?]
Always (when origin is empty)

[ahamlat]

Fixed with #6988