`parse` fails to parse empty files with `{ loc: true }` option

[fernandomg]

Hello Federico,

First of all, thank you so much for this fantastic parser!

The issue I'm facing has to do with the parse function and the loc option set to true, if I pass a file without parseable solidity code in it parse throws an error.

I tested the same scenario with tokenize and it worked fine.

Here is the code to reproduce the scenarios:

const parser = require('solidity-parser-antlr')

const emptyFile = ''

const commentedFile = `/*
pragma solidity ^0.5.3;

contract A {
  uint a;
  function b() {}
}
*/
`

const contracts = [{ emptyFile }, { commentedFile }]

const processFile = (parserModule = 'parse') => (loc = false) => contract => {
  const contractName = Object.keys(contract)
  const label = `\n\n=== ${parserModule.toUpperCase()} ${contractName} with ${JSON.stringify({ loc })} ===\n`

  try {
    const result = parser[parserModule](contract[contractName], { loc })
    console.log(label, JSON.stringify(result, null, 2))
  } catch (e) {
    console.log(label, e)
  }
}

const parse = processFile()
contracts.forEach(parse(true))
contracts.forEach(parse())

const tokenize = processFile('tokenize')
contracts.forEach(tokenize(true))
contracts.forEach(tokenize())

And this is the output of running it:

=== PARSE emptyFile with {"loc": true} ===
 TypeError: Cannot read property 'line' of null
    at ASTBuilder._loc (/**/solidity-parser-antlr/dist/ASTBuilder.js:1135:22)
    at ASTBuilder.meta (/**/solidity-parser-antlr/dist/ASTBuilder.js:1149:29)
    at ASTBuilder.createNode (/**/solidity-parser-antlr/dist/ASTBuilder.js:1158:34)
    at ASTBuilder.visit (/**/solidity-parser-antlr/dist/ASTBuilder.js:1187:15)
    at Object.parse (/**/solidity-parser-antlr/dist/index.js:83:21)
    at contract (/**/parse_fails.js:24:40)
    at Array.forEach (<anonymous>)
    at Object.<anonymous> (/**/parse_fails.js:33:11)
    at Module._compile (internal/modules/cjs/loader.js:816:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:827:10)


=== PARSE commentedFile with {"loc": true} ===
 TypeError: Cannot read property 'line' of null
    at ASTBuilder._loc (/**/solidity-parser-antlr/dist/ASTBuilder.js:1135:22)
    at ASTBuilder.meta (/**/solidity-parser-antlr/dist/ASTBuilder.js:1149:29)
    at ASTBuilder.createNode (/**/solidity-parser-antlr/dist/ASTBuilder.js:1158:34)
    at ASTBuilder.visit (/**/solidity-parser-antlr/dist/ASTBuilder.js:1187:15)
    at Object.parse (/**/solidity-parser-antlr/dist/index.js:83:21)
    at contract (/**/parse_fails.js:24:40)
    at Array.forEach (<anonymous>)
    at Object.<anonymous> (/**/parse_fails.js:33:11)
    at Module._compile (internal/modules/cjs/loader.js:816:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:827:10)


=== PARSE emptyFile with {"loc": false} ===
 {
  "type": "SourceUnit",
  "children": []
}


=== PARSE commentedFile with {"loc": false} ===
 {
  "type": "SourceUnit",
  "children": []
}


=== TOKENIZE emptyFile with {"loc": true} ===
 []


=== TOKENIZE commentedFile with {"loc": true} ===
 [
  {
    "type": "Keyword",
    "value": "/*\npragma solidity ^0.5.3;\n\ncontract A {\n  uint a;\n  function b() {}\n}\n*/",
    "loc": {
      "start": {
        "line": 1,
        "column": 0
      },
      "end": {
        "line": 1,
        "column": 73
      }
    }
  }
]


=== TOKENIZE emptyFile with {"loc": false} ===
 []


=== TOKENIZE commentedFile with {"loc": false} ===
 [
  {
    "type": "Keyword",
    "value": "/*\npragma solidity ^0.5.3;\n\ncontract A {\n  uint a;\n  function b() {}\n}\n*/"
  }
]