graph codegen: fails with nameless fallback functions

[YaroShkvorets]

As reported by @marcusrein

Reproduce:

• graph init with contract 0x760AfE86e5de5fa0Ee542fc7B7B713e1c5425701 on monad-testnet
• graph codegen

- Load subgraph from subgraph.yaml
✖ Failed to load subgraph from subgraph.yaml: Cannot read properties of null (reading 'map')
error Command failed with exit code 1.

The issue is with fallback function that doesn't have a name.
Should assign some default name for such nameless functions or just filter them out from ABI.

Sample ABI:

[
  {
    "inputs": [
      {
        "indexed": true,
        "internalType": "address",
        "name": "src",
        "type": "address"
      }
    ],
    "name": "Withdrawal",
    "outputs": null,
    "stateMutability": "",
    "type": "event"
  },
  {
    "inputs": null,
    "name": "",
    "outputs": null,
    "stateMutability": "payable",
    "type": "fallback"
  },
  {
    "inputs": [{ "internalType": "uint256", "name": "wad", "type": "uint256" }],
    "name": "withdraw",
    "outputs": [],
    "stateMutability": "nonpayable",
    "type": "function"
  }
]