perf(serializer): avoid addComma when not necessary

[cesco69]

Just a small performance improvements when an object contains only one key or an array only one item, the fjs code avoid to check to addComma

Currently this schema

fjs({
    title: 'additionalProperties',
    type: 'object',
    properties: {
      foo: {
        type: 'string'
      }
    },
    additionalProperties: false
})

produce this fjs code

function anonymous0 (input) {
      const obj = (input && typeof input.toJSON === 'function')
    ? input.toJSON()
    : input

      if (obj === null) return JSON_STR_EMPTY_OBJECT

      let json = JSON_STR_BEGIN_OBJECT
let addComma = false

      const value_foo_0 = obj["foo"]
      if (value_foo_0 !== undefined) {
        !addComma && (addComma = true) || (json += JSON_STR_COMMA)
        json += "\"foo\":"

        if (typeof value_foo_0 !== 'string') {
          if (value_foo_0 === null) {
            json += JSON_STR_EMPTY_STRING
          } else if (value_foo_0 instanceof Date) {
            json += JSON_STR_QUOTE + value_foo_0.toISOString() + JSON_STR_QUOTE
          } else if (value_foo_0 instanceof RegExp) {
            json += asString(value_foo_0.source)
          } else {
            json += asString(value_foo_0.toString())
          }
        } else {
          json += asString(value_foo_0)
        }

      }

    return json + JSON_STR_END_OBJECT

    }

whith this PR

function anonymous0 (input) {
      const obj = (input && typeof input.toJSON === 'function')
    ? input.toJSON()
    : input

      if (obj === null) return JSON_STR_EMPTY_OBJECT

      let json = JSON_STR_BEGIN_OBJECT

      const value_foo_0 = obj["foo"]
      if (value_foo_0 !== undefined) {

        json += "\"foo\":"

        if (typeof value_foo_0 !== 'string') {
          if (value_foo_0 === null) {
            json += JSON_STR_EMPTY_STRING
          } else if (value_foo_0 instanceof Date) {
            json += JSON_STR_QUOTE + value_foo_0.toISOString() + JSON_STR_QUOTE
          } else if (value_foo_0 instanceof RegExp) {
            json += asString(value_foo_0.source)
          } else {
            json += asString(value_foo_0.toString())
          }
        } else {
          json += asString(value_foo_0)
        }

      }

    return json + JSON_STR_END_OBJECT

    }

Diff

Side note: the benchmark don't cover this case

[mcollina]

lgtm