html formatter error when diffing with an array with undefined values

[DVLP]

The array values are undefined deliberately

html.format is giving this

Diff returned by jsondiffpatchinstance.diff

It could be because formatValue in HtmlFormatter is expecting JSON.stringify to return a string but for undefined JSON.stringify returns undefined

[benjamine]

pushing a small fix to prevent the error, although I think having undefined values could break your diffs in other unexpectable ways.