[date formatter] Display hyphen on null or undefined dates

[jbudz]

Currently in master and 4.2, null dates display an empty string while strings show a -.

Some test data if it's useful:

curl -XPUT 'http://localhost:9200/foo/bar/1' -d '
{
    "string1": "hello",
    "string2": null,
    "date1": "2011-12-15",
    "date2": null
}'

Closes #4891

[BigFunger]

LGTM