Make Dijkstra genesis config optional

[Minhcardanian]

• # Description

I found that cardano-node configuration parsing required DijkstraGenesisFile whenever ExperimentalHardForksEnabled = true, even though protocol instantiation already handled a
missing Dijkstra config by falling back to the empty Dijkstra genesis.

That made some experimental configurations fail at parse/startup time even though the runtime path already supported the absence of a Dijkstra genesis file.

In this PR I make DijkstraGenesisFile optional during config parsing so parse-time behavior matches the existing runtime fallback:

• if DijkstraGenesisFile is present, it is parsed and used as before
• if it is absent, parsing now keeps the Dijkstra configuration as Nothing

I reproduced the issue in the 10.7.0 worktree before making the fix, then verified the fix with:

• property-style parser regression coverage in cardano-node/test/Test/Cardano/Node/POM.hs
• cardano-testnet golden default config verification
• cardano-testnet dump/load integration verification

Validation used:

• nix develop . -c cabal test cardano-node-test
• nix develop . -c cabal test cardano-testnet-golden --test-options '-p "Golden tests.golden_DefaultConfig"'
• nix develop . -c cabal test cardano-testnet-test --test-options '-p dumping'

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated. These may include:
  • golden tests
  • property tests
  • roundtrip tests
  • integration tests
    See Running tests for more details
• Any changes are noted in the CHANGELOG.md for affected package
• The version bounds in .cabal files are updated
• CI passes. See note on CI. The following CI checks are required:
  • Code is linted with hlint. See .github/workflows/check-hlint.yml to get the hlint version
  • Code is formatted with stylish-haskell. See .github/workflows/stylish-haskell.yml to get the stylish-haskell version
  • Code builds on Linux, MacOS and Windows for ghc-9.6 and ghc-9.12
• Self-reviewed the diff