Netlify validator errors on null

[swyxio]

Describe the bug

swyxio/netlify-plugin-search-index#18

With this configuration:

[[plugins]]
  package = "netlify-plugin-search-index"
    [plugins.inputs]
    generatedFunctionName = "devopsdaysSearch"
    publishDirJSONFileName = null

The following error comes from Netlify:

10:21:00 PM: Failed during stage 'Reading and parsing configuration files': 
When resolving config file /opt/build/repo/netlify.toml:
Could not parse configuration file
Expected "'", "'''", "+", "-", "[", "\"", "\"\"\"", "_", "false", "true", "{", [ \t] or [0-9] but "n" found.

i believe the source is in netlify build's validation.

[ehmicky]

null is not a valid type in TOML (see toml-lang/toml#30, toml-lang/toml#146).

> require('toml').parse('test = null')
Uncaught:
[SyntaxError: Expected "'", "'''", "+", "-", "[", "\"", "\"\"\"", "_", "false", "true", "{", [ \t] or [0-9] but "n" found.] {
  expected: [
    { type: 'literal', value: "'", description: `"'"` },
    { type: 'literal', value: "'''", description: `"'''"` },
    { type: 'literal', value: '+', description: '"+"' },
    { type: 'literal', value: '-', description: '"-"' },
    { type: 'literal', value: '[', description: '"["' },
    { type: 'literal', value: '"', description: '"\\""' },
    { type: 'literal', value: '"""', description: '"\\"\\"\\""' },
    { type: 'literal', value: '_', description: '"_"' },
    { type: 'literal', value: 'false', description: '"false"' },
    { type: 'literal', value: 'true', description: '"true"' },
    { type: 'literal', value: '{', description: '"{"' },
    { type: 'class', value: '[ \\t]', description: '[ \\t]' },
    { type: 'class', value: '[0-9]', description: '[0-9]' }
  ],
  found: 'n',
  offset: 7,
  line: 1,
  column: 8
}

Instead, an empty string can be used if you would like user to override a default value.